I'm new to careing about serial numbers but, I have a guess since I now have a Yugo M59 with all numbers correct and a Yugo M5966A1 that I just bought from Weidners. So, here is my guess based on My Yugo M5966A1. I noticed all serial numbers matched but there was this scribed in number on the trigger guard which did not match any other numbers. After I took the barrel assembly out of the stock to clean up everything, I noticed the bottom of the barrel had a serial number that matched the hand scribed number. Since it has been said by many that many Yugo M59 and newer model M5966's went through a re-arsenal process to convert them to a M5966A1 or A2, I presumed the barrel was changed out and the new barrel serial number was hand scribed on the trigger guard. I can not say for sure this is true. It just made since to me for two reasons. First if one changes out a barrel it must be timed for the bolt & bolt carrier but you don't need to change anything else unless another part is worn (I.E. bolt carrier or bolt). This would explain why my M5966A! had all matching numbers except the barrel and others have all matching number except barrel, bolt carrier, etc,etc. Second a Yugo barrel is non chromed lined and after shooting corrosive ammo they would most likely need replacing sooner or later. I'm going through this process on the M59 I have. Sportsman Guide.com has Yugo SKS barrels for the M5966 for $39.99 but they have a threaded barrel and I need all the dimensions for a M59 barrel to convert it. If you can build a AR15 or AK you can change out the barrel. If you have not done so, it will cost you a bunch from a qualified gun smith to do this.