different collapsible sonnar 5cm f2 no serial number

I have a collapsible sonnar 5cm f2 made without filter threads uncoated only marked sonnar without a serial number I had it cleaned inside the barrel its marked with an upside down v and the number 420 looking for a zeiss historian to shed some light on this lens

So what does the back look like?

The retaining ring on the rear element is different then others

It looks like the mounting and aperture ring are normal, but the optical capsule is very different from mine, a 1936 version. Maybe a prototype that escaped? It looks like something is missing on the front mount in front of the aperture ring, and the name ring has tightening slots in it unlike mine.