Hello alexg, this mark is one of the very few legit postal bajonet makers and to my best knowledge this mark does not occur on socalled normal walk out bayonets (if I am wrong please one of the bayonet experts may correct me). So I would tend to believe that this is an original DRP stamping-
BUT the bayonet is -imho- in very bad condition. These bayonets do come by from time to time (I think to remember one currently at a dealer, if you are interested please send me a pm) and I personally would wait for a better one.
If eventually a resale is necessary, which can happen to all of us collectors, condition is the most worthwhile aspect!
Regards,

wotan, gd.c-b#105

"Never look for sqare eggs" as a late owner of an original FHH-dagger used to say.