I have an underwood with the exact same marking in the same place. Mine did not have it on the stock either. I believe they came back from Italy , but they probably went through a rebuild and lost the stock to another gun or it was changed somewhere in it's life . In my mind I don't think it changes the value like an im
It's a sign that the carbine was a return to the US Army from Italy which turned it over to and was sold by the CMP. I still think someone has tried to restore your carbine and has replaced some of the parts like the stock. I do not think that was the stock on it if sold by the CMP.
I think you are right. The rifle came with a CMP certificate of authenticity dated 2008. I bet one of the previous owners switched stocks. Also, it came with a repro Type 1 rear sight that I plan to replace (ref. my other post in this forum) . Who knows what else was switched. By the way, did all Italian returns go through the CMP?