Though this system works fine for the Eibar-based Star, use care when trying to proof-date a gun by another Spanish maker.Other regions have different proof houses with different marks and codes.
If the weapon survives a certain number of these, it passes, and all weapons in the lot get the proof mark to certify they meet the government's minimum expectations of function and safety.
Though the US government has inspection processes for military goods, there is no proof house of any sort for domestic US gun makers.
Proof marks are often very shallow, and in obscure places, like far up the trigger guard.
You might not notice them unless looking, under bright light, at low angles. They may be under the stocks, and on small guns, very often are completely or partly under the grip panels.
Although many other countries have military acceptance marks as well, I do not know their design; if anyone does know for sure, please send me a good quality photo or other reference information.
The three proof marks indicate receipt at the proof house, passing of proof, and the date of proofing.
There are 3 on a typical commerical arm, and four on a typical military arm.
Note that most will not be hilited in white as these are. The Spanish government set up a proof house in Eibar starting in 1923.
Supposedly, this is: However, I have not seen this that I noticed, and a perusal of the few post-1995 guns in my possession doesn't seem to indicate it, and of photos doesn't help either.