The only thing I know is that the lots of 336's were stamped RC until 68 or 69 I think. Is your serial number on the top tang or the lower tang? After the Gun Control act of 68 all serial numbers were moved to the top tang so it couldn't be removed from the receiver. If your number is on the top maybe it is a late production 68 built after the law went into effect. You never know what went on with them.