Question: What is bass player Stuart Sutcliffe often referred to, thanks to his presence on a band's early demo recordings?

full_context: Document 1: Stuart Sutcliffe: Stuart Fergusson Victor Sutcliffe (23 June 1940 – 10 April 1962) was a British painter and musician best known as the original bass guitarist for the Beatles. Sutcliffe left the band to pursue his career as a painter, having previously attended the Liverpool College of Art. Sutcliffe and John Lennon are credited with inventing the name, "Beetles", as they both liked Buddy Holly's band, the Crickets. The band used this name for a while until Lennon decided to change the name to "the Beatles", from the word "beat". As a member of the group when it was a five-piece band, Sutcliffe is one of several people sometimes referred to as the "Fifth Beatle."
Document 2: Mother (John Lennon song): "Mother" is a song by English musician John Lennon, first released on his 1970 album "John Lennon/Plastic Ono Band". An edited version of the song was issued as a single in the United States on Apple Records, on 28 December 1970. The single runs about 1:41 shorter than the album due to a lack of the tolling bells intro and a quicker fadeout. The B-side features "Why" by Yoko Ono. The song peaked in the US at number 19 on the "Cashbox" Top 100 and number 43 on the "Billboard" Hot 100.
Document 3: Fifth Beatle: The fifth Beatle is an informal title that various commentators in the press and entertainment industry have applied to people who were at one point a member of the Beatles, or who had a strong association with the "Fab Four" (John Lennon, Paul McCartney, George Harrison, and Ringo Starr) during the group's existence. The "fifth Beatle" claims first appeared in the press immediately upon the band's rise to global fame in 1963–64. The members have offered their own beliefs of the "fifth Beatle":
Document 4: Bad to Me: "Bad to Me" is a song credited to Lennon–McCartney. In late interviews, John Lennon said that he wrote it for Billy J. Kramer with The Dakotas while on holiday in Spain. However, in a 1964 interview he said that he and Paul McCartney wrote it in the back of a van, so Paul contributed to it. Billy J. Kramer with The Dakotas released their recording of the song in 1963 and it became their first number 1 in the UK Singles Chart. Paul McCartney was present during the recording session at Abbey Road Studios. The single would be released in the US the following year, and become a top-ten hit there, reaching number 9. It became one of the first occasions a Lennon–McCartney composition made the US Top 40 recorded by an artist other than the Beatles (the first being "A World Without Love" by Peter & Gordon; another being "Goodbye" by Mary Hopkin).
Document 5: Hello Little Girl: "Hello Little Girl" is the first song written by John Lennon, and credited to the Lennon–McCartney songwriting partnership. According to Lennon, he drew on an old "Thirties or Forties song" that his mother sang to him. Written in 1957, it was used as one of the songs at the Beatles unsuccessful Decca audition in 1962. They recorded a home demo of it, with Stuart Sutcliffe on bass, which is available only on bootleg currently.
Document 6: You're Going to Lose That Girl: "You're Going to Lose That Girl" is a song by the Beatles from the album and film "Help!", written by John Lennon and Paul McCartney. Capitol Records originally titled the song "You're Gonna Lose That Girl" in the United States.
Document 7: Girl (Beatles song): "Girl" is a song written by John Lennon (credited to Lennon–McCartney) and performed by the Beatles on their 1965 album "Rubber Soul". "Girl" was the last complete song recorded for that album. "Girl" is one of the most melancholic and complex of the Beatles' earlier love songs.
Document 8: Andy White (drummer): Andrew "Andy" White (27 July 1930 – 9 November 2015) was a Scottish drummer, primarily a session musician. He was affectionately christened "the fifth Beatle" as he is best known for replacing Ringo Starr on drums on the Beatles' first single, "Love Me Do". White was featured on the American 7" single release of the song, which also appeared on the band's debut British album, "Please Please Me". He also played on "P.S. I Love You", which was the B-side of "Love Me Do".
Document 9: I Saw Her Standing There: "I Saw Her Standing There" is a song by the English rock band the Beatles written by Paul McCartney and John Lennon. It is the opening track on the band's 1963 debut album "Please Please Me".
Document 10: Stuart Sutcliffe: Stuart Fergusson Victor Sutcliffe (23 June 1940 – 10 April 1962) was a British painter and musician best known as the original bass guitarist for the Beatles. Sutcliffe left the band to pursue his career as a painter, having previously attended the Liverpool College of Art. Sutcliffe and John Lennon are credited with inventing the name, "Beetles", as they both liked Buddy Holly's band, the Crickets. The band used this name for a while until Lennon decided to change the name to "the Beatles", from the word "beat". As a member of the group when it was a five-piece band, Sutcliffe is one of several people sometimes referred to as the "Fifth Beatle."
Document 11: The Beatles: The Beatles were an English rock band formed in Liverpool in 1960. With members John Lennon, Paul McCartney, George Harrison and Ringo Starr, they became widely regarded as the foremost and most influential act of the rock era. Rooted in skiffle, beat and 1950s rock and roll, the Beatles later experimented with several musical styles, ranging from pop ballads and Indian music to psychedelia and hard rock, often incorporating classical elements and unconventional recording techniques in innovative ways. In 1963 their enormous popularity first emerged as "Beatlemania", and as the group's music grew in sophistication in subsequent years, led by primary songwriters Lennon and McCartney, they came to be perceived as an embodiment of the ideals shared by the counterculture of the 1960s.
Document 12: Lennon–McCartney: Lennon–McCartney was the songwriting partnership between English musicians John Lennon (9 October 19408 December 1980) and Paul McCartney (born 18 June 1942) of the Beatles. It is one of the best known and most successful musical collaborations in history, with the Beatles selling over 600 million records, tapes and CDs as of 2004. Between 1962 and 1969, the partnership published approximately 180 jointly credited songs, of which the vast majority were recorded by the Beatles, forming the bulk of their catalogue.
Document 13: The Beatles in Hamburg: The Beatles members John Lennon, Paul McCartney, George Harrison, Stuart Sutcliffe and Pete Best regularly performed at different clubs in Hamburg, Northern Germany, during the period from August 1960 to December 1962; a chapter in the group's history which honed their performance skills, widened their reputation, and led to their first recording, which brought them to the attention of Brian Epstein.
Document 14: Hello, Goodbye: "Hello, Goodbye" is a song by the English rock band the Beatles, written by Paul McCartney and credited to Lennon–McCartney. Backed by John Lennon's "I Am the Walrus", it was issued as a non-album single in November 1967, the group's first release since the death of their manager, Brian Epstein. The single was commercially successful around the world, topping charts in the United States, the United Kingdom, Canada, Australia and several other countries.
Document 15: Pete Best: Randolph Peter "Pete" Best (born Randolph Peter Scanland, 24 November 1941) is an English musician, principally known as an original member and first drummer of the Beatles, from 1960 to 1962. He has been referred to as the Fifth Beatle.
ground_truth: Fifth Beatle