DWM 1915 hold-open

i bought a 1915 DWM, but i am still waiting for my permission to take it home :-(. it is a magnificent gun. i bought it at a little local gunshop. it was sold by an old widow, her man owned it since ww I. it has always been in privat hands, hasn't been in ww II! it is in original ww I condition, not reworked. the firing-pin has no (gas-channels?). it is completely matching, also the original wood-bottom mag! the sn is 358. there are the original wooden-grips, covered by old, old grease. the gun made only a few shots, the firing pin looks like new (after i removed very old grease) and the blueing has no damage anywhere. i wonder how this gun could have the hold-open with a wrong number (78), it is looking 100% original, was also covered with the same old grease. is it possible, that it was a numbering-mistake by the manufacturer? I can't believe, that this is a spare-part, it seems to be there, since it was built. there are some rust-markings at those places, where a luger stays in its holster (the holster is not there..:-(). are there other dwm-guns with mismatching hold-opens?