The list is endless as to why this is wrong, but in short no SDP rifle is marked this way, the BC is entirely fabricated, they are not done this way, second it has no characteristics of the first block of SDP 1944 production, it lacks the proper acceptance and it should have two FP and be SRO, not SBO, - this pattern didn't occur until later in 1944.
It can't be SS contract because it is the wrong receiver pattern and it isn't marked properly for one anyway. They should have slapped a rune on it and at least be plausible marking wise for one of the variations,- it wouldn't have fooled anyone but at least it could have been plausible for a novice.
Anyway, I doubt it has a single original part on it, but it hardly matters as the rifle is entirely incorrect regardless of what angle you look at it from. Anyone who has bothered to study SDP to any degree would know this, that is why it sold for so little compared to an original rifle, the humper didn't not know much about SDP production.