Variations.
 

The first and really, the only explanation of MOS was given to me when I was about 7 years old and tagging along with my Dad who worked as a camera assistant for Gerald Hirschfeld and Owen Roizman at MPO VideoTronics in New York City.

MPO was a major commercial house in the City and cranked out hundreds of commercials a year shooting on equipment such as Mitchell BNCs and Technicolor 3 Stripe cameras.

The explanation for MOS was given to me by the In-House camera repairman, Willie Berge, (pronounced BURGEE), who was a German with a heavy accent.

His explananation was the original one I cited, and considering the source, I'm sticking to it!

The other possibilities I cited were from a website that I had seen before, although I had heard the "Minus Optical Stripe" before.

RB

The "Mit Out Sound" explanation is the one I have always heard.

Me, too.

MOS
 

It seems I remember MOS being to taught to me as meaning Mute or Silent