Shipbucket

For what it's worth Wikipedia.de mentions an early-50s (pre-Bundeswehr) involving a rebuild of the foredeck superstructure to be fitted for but not with a 20mm gun.
This is sourced to this book, without a direct quote: Hanns Postel,. Der Seegrenzschutz. In: Hartmut Klüver (Hrsg.): Stationen deutscher Marinegeschichte (II): Deutsche Seeverbände 1945–1956.

Here's the full quote of the relevant part, in German (highlight mine): This might imply that the 20mm gun was fitted during or around the handover from the Seegrenzschutz to the Bundesmarine. The Decca radar is also mentioned as part of this 50s refit.
The "W 6" and "W10" markings correlate in this (apparently official) ships list with the KFKs entering Bundesmarine service in 1956 under Class 386. Not that there was much doubt about that, but it fits with the tentative timeline (post-'56 marking, post-refit radar and gun added post-'56).

Given the dates, I would see the gun more as a Hispano-Suiza 820 aka MK20-1, which was in German service earlier than the Rh.202. Can't seem to find pictures of that one in naval mountings to compare, though.

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So, the gun would be a 20mm indeed and likely a Hispano-Suiza 820 or Rheinmetall Rh 202?
I will check out if I can find more, regards these two) Its a lot of info I did not have)
Thanks a lot so far guys!))