The rifle (barreled receiver) looks fine to me, this rifle variation is not SDP made, these are rifles made from components that were supplied to the KM (navy) and that were subsequently assembled into rifles. Therefore these rifles come in many different forms with slight variations in make up and how they are marked. The serialing format can vary widely, often they have "m" prefixes and often "N" suffixes, one can assume m=marine or navy, with N=North Sea. But several have neither a prefix or suffix like this one.
One would have to see the stock to know if this is an original stock, which is incredibly rare to find for this variation. Almost all do not have original stocks, but I would think the stock would not have the normal G.29/40 markings, as these were not made by Steyr, so they would have a ordnance depot pattern to them.
As for the KM relationship with this rifle, that is obvious, the G.29/40 was from beginning to end destined to the KM, the only reason some went to the Army (Heer) is because SDP had to divert some to make up for shortfalls in production.
Bottom line, the OP needs to a lot more pictures, but this style of FP is typical of the variation and the abnormalities of the serial fonts is not inconsistent with this variation.