Admittedly, the photos show sloppy workmanship which is uncharacteristic of the Germans. I'm not qualified to judge these by any means, I'm just making comparisons. Obviously if taking a $200 rifle and turning it into one worth thousands is just a matter of some stamps, great care has to be taken.
As for the numbers not matching, being built that way seems very unlikely. Why bother making the effort to stamp something and then not put it together. The book says the SN is on the barrel, the bolt top, the bolt sleeve, the cocking piece and the safety.