I purchased a SKS and I'm not sure quite what it is. The receiver is stamped 2 M21 N 5754 and the mag, trigger guard, carrier and cover are stamped H5754. I was under the impression that lettered rifles had the same serial number on the receiver. I would believe matching parts were placed on a different receiver, but the "5754" numbers match. Any idea? I have not seen this before and can't find any information on it.