neoinarien -
You would want to make sure the disk is not a fake, and below are some comments by someone who would be considered an expert on Finnish Mosin Nagants.
Mauser720 - Ron
"These discs were dropped on most rifles around the time of the Winter War, as rifles that were captured gave too much information to the enemy. Some disks did survive but it is not common to locate rifles that still have these in place. It is most common to see such discs in M27 rifles. One might also see such a filled hole in areas of the stock where that could not have had a stock disc. In most cases this is a Finnish repair of stock damage. The repair is very common and almost looks like dark wood putty.
"Did the Finns use stock discs post war?
"Yes they did but again the use was not common. War era discs will be brass in look and color while the post war discs look like aluminum. In many cases these post war discs are artillery units the most common being 2.PSTO KAR PR. which is the 2nd Karelian Artillery Brigade these discs as far as I know being seen only on M39's. I have never seen a wartime M39 stock disc as it seems the Finns banned the use of these discs before M39 production got underway as a security measure. What is not known is why the practice was once again undertaken post war."
http://mosinnagant.net/finland/finnfaq.asp