I've seen Sigi pieces where part of the markings were added as later inscriptions, but this is the first piece I've seen where all of the marks have been inscribed. However, I have no problem in accepting them as genuine.
Many people assume that the marks are the last additions in the creative sequence, but in an assemblage like this piece they are one of the first steps, as it is not possible to stamp the piece after assembly.
If the maker forgot to mark it early on, or the marks were later obliterated or obscured in the soldering or polishing processes, inscription after its completion would make sense.
There is no doubt that, genuine or not, marks like these tend to cause insecurity. However, the presence of the eagle mark with a known Sigi number should provide reassurance.