I have seen many K31s having a Mo marked bolt cap, with 42 or 43 date stamps. This 1944 K31 has the bolt sleeve also Mo marked. The rifle has a "51" stamped on the tang indicating it had gone through arsenal refurbishment in 1951. It looks like the arsenal only replaced the barrel during the refurbishment and wasn't bothered to replace the bolt sleeve. I believe that the vast majority of the Mo marked bolt sleeves had been replaced, I am a little surprised this one still has the original Mo marked bolt sleeve.