Stamp or not wouldn't worry me in the slightest.
Whether the gun is smooth accurate and of sufficient power for my needs is what matters. If yours (your friend's) is then nothing to worry about. If it isn't and it was bought new then return it to the dealer who is responsible for putting it right or replacing it.
It is unlikely to go wrong later (except by misuse), but if it does so once the warranty has lapsed, there are plenty of spares available and willing individuals out there to put it right if you/your friend cannot do so themselves.
Happy Shooting!! Paul.
"We cannot solve our problems with the same thinking that we used when we created them" - Albert Einstein.