I have found some Mark 1*** rifles with new serial numbers and the prefix CR, ER and G
and I even found two Mark 1** IP rifles converted to Mark 111, interesting that the IP rifles retain their original receiver serial number and the new barrels are numbered to match.

Most difficult (for me) is to find a Mark 1*** still with the original serial number such as my BSA 1906 in the photo. (barrel matches too)