This was bought in a charity shop for the lens, an Industar f3.5 50mm unit, which can be seen in various Zenit galleries and on the ASAHI pentax S1a elsewhere on this site. The 'B also came with the Helios M44-2 lens, as did most of the early Zenits, so bodies can be found with either lens
The body is as simple as they come, being the same as the 'E, but without light meter, shutter speeds to 1/500th sec and the standard M42 lens mount it seems to work well.
The main changes from the 3M are a 'level' top with no step down for the wind on lever, M42 lens mount and an instant return mirror.
I'll run a film through it and post an update when that's done.