Its the last three digits of the serial number of the rifle. Probably used to keep bolts and receivers together during the manufacturing process.
It doesn't bother me since its on the underside of the bolt handle and you'll never really see it without actually looking for it.
Sounds like the distance between the action and DBM is not correct and the magazine hits the action before it can lock into place.
Maybe you could try placing thin washers between the DBM and the stock to get the correct distance.
