Question: Who was the sole constant member of Electric Light Orchestra who collaborated with Del Shannon with on the Rock On! album?

Context articles: Showdown (Electric Light Orchestra album)
Electric Light Orchestra Live
The Electric Light Orchestra (album)
Violinski
Jeff Lynne
The Very Best of the Electric Light Orchestra
Rock On! (Del Shannon album)
Richard Tandy
The Essential Electric Light Orchestra
ELO 2

Showdown is an Electric Light Orchestra (ELO) compilation album, covering their Harvest Records years.
 It is the first in a very long line of Electric Light Orchestra compilation albums.
 It was also the first time the hit single "Showdown" featured on an album in the UK.
Electric Light Orchestra Live is a live album by Electric Light Orchestra.
 It was released on 19 April 2013 in the UK, and on 23 April in the US on Frontiers Records.
The Electric Light Orchestra is the eponymous debut studio album by English rock band Electric Light Orchestra (ELO), released in December 1971 in the United Kingdom by Harvest Records.
 In the United States, the album was released in early 1972 as No Answer, after a misunderstood telephone message made by a United Artists Records executive asking about the album name; the caller, having failed to reach the ELO contact, wrote down "no answer" in his notes, and this was misconstrued to be the name of the album.
Violinski was a rock band formed in 1977 by Electric Light Orchestra member Mik Kaminski on violins, and former member Mike de Albuquerque on guitar and vocals; plus Barry Dunnery (formerly of Necromandus), John Hodgson (drums), Paul Mann, John Marcangelo (keyboards) and Iain Whitmore (bass guitar and vocals), formerly with Starry Eyed And Laughing.
 At one point, former Wizzard member Bob Brady was lead vocalist, and Rod Butler, who played lead guitar for a while, also appeared on the BBC show, "Top of the Pops".
 At the time Kaminski was still a member of the Electric Light Orchestra, who were then enjoying their greatest success.
 John Hodgson had been a member of Rick Wakeman's band, the English Rock Ensemble, performing on percussion.
 He appeared on "The Myths and Legends of King Arthur and the Knights of the Round Table" and "Lisztomania" albums.
 He toured with Wakeman to North and South America, Japan, Australia and New Zealand during the period 1974-1976.
Jeffrey Lynne (born 30 December 1947) is an English songwriter, composer, arranger, singer, multi-instrumentalist and record producer who gained fame in the 1970s as the lead singer and sole constant member of Electric Light Orchestra.
 In 1988, under the pseudonyms Otis Wilbury and Clayton Wilbury, he co-founded the supergroup Traveling Wilburys with George Harrison, Bob Dylan, Roy Orbison and Tom Petty.
The Very Best of Electric Light Orchestra is a compilation album by the Electric Light Orchestra (ELO), released in 1994.
 It peaked at number four on the UK Albums Chart.
Rock On!
 is the 12th studio album by American rock singer Del Shannon, released posthumously in 1991.
 Shannon returned to the studio after a long hiatus, collaborating with Jeff Lynne of the Electric Light Orchestra, Mike Campbell, Tom Petty, other members of Tom Petty and the Heartbreakers, and Richard Greene.
 Shannon committed suicide on February 8, 1990, and the album was completed for release on October 1, 1991.
Richard Tandy (born 26 March 1948) is an English musician, best known as the keyboardist in the rock band Electric Light Orchestra ("ELO").
 His palette of keyboards (including Minimoog, Clavinet, Mellotron, and piano) was an important ingredient in the group's sound, notably on the albums "A New World Record", "Out of the Blue", "Discovery", and "Time".
 Tandy was inducted into the Rock and Roll Hall of Fame on 7 April 2017 as a member of Electric Light Orchestra.
The Essential Electric Light Orchestra is a single CD, US-only compilation album by the Electric Light Orchestra (ELO), released in 2003.
 It is part of Sony Music's "The Essential" series.
 It was preceded by US compilations "Olé ELO" in 1976 and "" in 1995.
ELO 2 is the eponymous second studio album by the Electric Light Orchestra (ELO), released in 1973.
 In the US, the album was released as Electric Light Orchestra II.
 It was also the last album to be released by the band under the Harvest label, the last where the band used the definite article "The" on its name and introduced their acronym ELO.