So after some consideration, I decided to adapt the back. The edge that
the holder butts up against isn't absolutely critical, so if I purchase
some other holders at some point that turn out to be a tad on the short
side and light is leaking in, I can always add another layer of tape to
the end of the holders to shim them up.
In the process of fitting the holders to the back, I did discover that the
S&S holders really aren't absolutely consistent in manufacture. Of the
four surfaces on the two holders, the distance from the lock rib to the
end of the holder was just a hair longer on one, making for a slightly
tighter fit, and requiring that I add a little more relief to the back to
accommodate that one side.
In the end, since the film depth and other dimensions seem to be the same, I suspect that the back is designed to the same spec as ANSI, even if the camera is pre-ANSI, because the standard had to have come from some pre-existing norm. It might just be that if the holders are a tiny bit off and the back has changed in some way with age, then some sort of adjustment is going to be necessary with a camera this old.