I'm only citing H. Thiele's reference book "Fabrikationsbuch Photooptik II, Carl Zeiss Jena 1927-1991"; Munich 2005. It is based upon the records kept by Carl Zeiss Jena.
For the Sonnar 2/5 cm there are only TWO entries (p. 325):
Sonnar 2/5cm: construction 04.04.33; 2 produced; serial 1436679,1436680 (08.08.33)
Sonnar 2/5cm: construction 01.12.33; 200 prod. ; serial 2805401-2805600 (13.09.47).
For statistical reasons, it is not very likely that one of the two from 1933 are offered.
Also for statistical reasons, it is not much more likely that one of the 200 post-war lenses is offered, either. Nevertheless, it is possible.
For the latter, it shouldn't be all too easy to identify a fake. Post-war Zeiss lenses tend to be made of lightweigth aluminium rather than of heavyweigt chrome-on-brass, but so do Russian fakes. Russian fakes are often within the Zeiss serial number range as the Russians took over the plant and the archives.
But there are hints. Black front ring: suspect fake. Large clumsy letters and numbers on front ring: fake. Large red focusing triangle with broad base: fake. Large "M" (and comparatively large numbers) on focussing scale: fake.
I feel that there are loads of fakes around compared to a very small amount of original lenses. If 202 lenses have been produced in wartime and immediate after-war times, the simple likelyhood of too many surviving until 60-70 years later is not too high. And why should they be offered on the market on a regular basis?
Yes, there have been lenses that were converted from Contax-mount to M39/LTM, but these were individual cases that are out of records.
That's the problem.
I found this link very helpful:
800 yrs Dresden
80 yrs Zeiss Ikon