Complex sentence: In 1958, as an offshoot of his magazines, Marks began making short films for the 8mm market of his models undressing and posing topless, popularly known as "glamour home movies".
Split into simpler sentences: In 1958, as an offshoot of his magazines, Marks began making short films of his models undressing and posing topless, for the 8mm market. These were popularly known as "glamour home movies".