I've done a bit more digging around and can at least add a bit of information, though I still have no answer. There was a suggestion that it was an identifier used by SJG Neuhausen (that is SIG) itself. That doesn't seem likely. The mark does not appear on any Swiss M1906 Pioneer bayonets (all of which were made and marked as such by SJG Neuhausen, just two years before production of Mondragon rifles). It also occurred to me that perhaps the mark would appear on the 4,250 Mannlicher 1893 rifles produced by SJG Neuhausen during 1895-1896 (W+F Bern took over production in 1896). I've not seen the mark in the few photos I've been able to find, though the photos are certainly not clear or close or good enough to actually say, one way or the other, and I don't have easy access to one. Does anyone own a Mannlicher 1893 rifle made by SJG Neuhausen and, if so, would you please carefully inspect it? If it is not there, either, then the suggestions that: (1) the mark is somehow associated with the original Mexican contract, or (2) it is the mark of a SJG Neuhausen subcontractor, are the most likely answers, at least to me. The second theory seems the less likely of the two though, because again we would expect to see the mark on other subcontracted parts of other guns and steel components made by SJG Neuhausen, and we do not. Moreover, the fact that the mark appears on all variants of the rifle (thank you Mondragon), and the entire production was at least originally intended for Mexico, seems to support the first theory, at least marginally more than the second. Did the buyer, i.e., the Mexican Government, put its own inspector at the factory in Neuhausen? If so, could it be that Mexican Government inspector's mark? That is an intriguing possibility.