In brief... there are questions as to whether the Lugers (mostly Model 1906s, there are a few 1900s as well) which are adorned with Crossed Rifles on the chamber were intended for a "Russian" contract or customers as identified by various authors and labeled as "Russian" years ago before many such guns were known. The Crossed Rifle markings are inconsistent and appear to be largely engraved rather than stamped. In the case of the 1906 "Russians", their safety markings and internal markings are identical to 1906 Bulgarian contract guns (although they are 9mm, not 7.65mm like the Bulgarians). They differ in the Cyrillic writing on the extractor. Some argue that both markings are Bulgarian and therefore they are all Bulgarian guns. Some (including Gortz and Sturgess) say they were manufactured in the same production run because so far, there are no overlaps in serials between the 1906 Crossed Rifle ("Russians") and 1906 Bulgarians.
http://luger.gunboards.com/showthrea...356#post303356
It makes a difference because the "Russian" guns seem to carry a much higher premium in the marketplace than the equally rare (actually rarer) 1906 Bulgarians, and several fakes have been identified.
I hope this is a fair, although brief, treatment of the subject. There are lots of threads on this at
luger.gunboards.com, some of which have been closed due to commentary that gets a little too personal.