Hi and welcome to the forum (at least to posting here!)
The plastic base was first disclosed on a Mauser drawing in February, 1940.
The "fxo" code was introduced in 1941. I also believe Haenel generally used aluminum bases through 1941. The plastic bases were in use from 1941 on. While WW-II vintage, I believe that they are too late to be a match your 1939.
It may be the flash, but the trigger and safety lever look strawed (golden in color). That process stopped in 1937 at the factory...
Marc
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 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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