I do not know when the date is added, but note that it is not exactly the date of manufacture, but the date of proof, which is always later.
Probably the same year though, so its a good guide.
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.
The bullet between instead of asterisk above is something I have not seen.
This is also a good example of how hidden proomarks can be.
They are usually on and/or near the front of the trigger guard.Though 100% of armor in those early days was tested, this rapidly changed to batch testing, which is what we have now with most firearms.A representative sample of each lot of weapons is tested with a proofing cartridge, which has vastly higher pressure than the normal specifications.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.