I'm not familiar with markings on CZ50's, but for CZ52's in the Czechoslovak Army the years other than the year of manufacture were often stamped when the pistol was refreshed at the arsenal. Since CZ50's were more often police pistols or border guard pistols, the police may have had an arsenal rebuilding system too, but it just speculation on my part. However, the CZ52's that underwent arsenal refurbishment have "VOZ" and the year of refurbishment stamped somewhere on the frame. Some heavily used pistols were refurbished several times. I don't know if your CZ50 has a mark like that, so you could look for it, but I just spent a little while looking at a few dozen high resolution photos of CZ50's on the internet and I don't see any marks like that on any of them.
Scott