thermostat:
I don't know what country you were operating in, but the 727 was the same as a L1011 in certification and FAA rules. The guy who sat sideways at that big, funny panel was required to have an appropriate F/E certificate. He did not have to have a pilot's certificate of any type whatsover.
Your company and/or union may have elected some additonal non-regulatory requirements for the F/E position. That often did happen. But, nonetheless, the FAA didn't recognize SO for that seat, other than perhaps on an ad hoc, local FAA basis.
I was captain on both types for a lot of time at TWA.