I used SOS a bit, but did anyone use Stopgap itself, or only its son?
I was going to ask the same question.
Funny, I still often (tend) to use line numbers to do multi-line operations not marks - probably because SOS was one of the first editors I learned and first I learned to use really well.
Now here is where my memory is hazy. I do remember using SOS on the PDP-10s which had mostly glass TTYs, but for some reason I have memories of using it on an ASR-33 on the TSS/360 - which must be wrong. I thinking, there probably had to have been an IBM editor similar to it that I'm confusing with the TOPS/TENEX. I also have no memories of what editor we used on the original VMS V1.0 machine -- it must have been a member of the TECO family, although again I want to say we had something like SOS.
I do remember that learning ed(1) on UNIX was a piece of cake and immediately loved regex for patterns. I was still doing some PDP-10 hacking for the CS-Dept in those days, and bitching that I wanted ed(1) on the 10s after I mastered it.
Clem