Since no one else has yet replied, I will simply say that I have seen a number of these letter prefixes, including "W." I have read that they stand for different purchase orders, which makes sense to me. That said, it is unusual AKAIK to see the prefix accompany the SN in multiple places, i.e. on the stock as well as the receiver. Without the crest, and if it is not completely "scrubbed," it is probably an early conversion at a time Yugoslaviawas trying to get rifles into the hands of troops as quickly as possible while spending as little time as possible reworking the rifles. So SN practices may have varied at that point - that is a complete guess on my part, and I am certainly no expert.