George,
I am certainly aware of the P.08 variations that left the factory (Mauser and Krieghoff) with magazines that had extruded bodies and a black plastic bottom and had a partial serial number pantographed on the body spine. These are well documented and legitimate variations.
However, I am not aware that Mauser ever shipped a 42 byf P.08 with an fxo magazine with plastic bottom (i.e. Type 6) that was serial numbered on the base. Serialization of magazines had stopped long before the Type 6 became standard issue. Even late issued Type 5 magazines with aluminum bodies shipped with 42 byfs were not serialized although you see a few on early production..
However, that is not to say that some well meaning armorer someplace faced with issuing a replacement magazine did not serialize them. Or for that matter any other person. I certainly have seen serialized fxo black bottom magazines; some well done and some not so much.
But, I am always willing to learn.
Respectfully,
John