different collapsible sonnar 5cm f2 no serial number

[randy6]

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

 

[outwest]

So what does the back look like?

 

[randy6]

The retaining ring on the rear element is different then others

 

[E. von Hoegh]

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.