My 1949 ZK Sonnar is almost identical to my late German LTM Sonnar.
From the thread I did on it on ziforums.com
No question that this lens is made from German parts. Some minor construction differences: NO set screw holding the rear optics fixture in place, no hidden set screw holding the name ring in place. Material is the same as the Wartime LTM Sonnars.
It looks like this lens started out as Zeiss SN 2726739.
The plastic case that it came in, compared with a J-3 case on the Left.
Lens compared with a Wartime Zeiss LTM Sonnar.
Serial Number, note that it looks like a J-3 SN.
The first ZK's had "1948" and then a SN.
Rear module has hand engraved numbers like a J-3. German LTM Sonnars do not have these extra markings, just the machine stamped lens SN.
Big "M" and two Set Screws as in a Wartime LTM Sonnar.
The larger of the tewo screws is the Stop for close-focus and infinity. The J-3 does not use this second set screw.
Not sure if the rear cap is original, but it probably is. Shows who made the lens, KMZ.
Optical Performance of the lens is on par with my 272 series Sonnar in LTM.
I just received my early 1953 KMZ Jupiter-3, and will be rebuilding it. German glass in a Russian mount.