The "s" editor is written by Webb Miller and appears in his book "A Software Tools Sampler." There was a discussion about it...here, or COFF, or cctalk, or the PiDP-11 mailing list....a few months ago. Someone had already done the lifting to make it classic v7-compatible, but I don't remember who.
Its interface is very close to vi, but it is quite compact and works well on v7.
Since I find ed thoroughly unpleasant to use, having a screen editor was a must for me to use v7 for any length of time, and s fills that role rather nicely.
Adam