Interesting Yugo barreled receiver!

[running-man]

I ran into this one while collating the March GB sales lists (click on image to enlarge):

S/N C41207 @ http://www.gunbroker.com/Auction/ViewItem.aspx?Item=549139880:


Clearly that gun is an M59/66A1 with a square backed GL rear ring and a non ported GL.  WTF is it doing on an early C block gun like that?!?!  It should be an M59 by everything I've ever seen...

Note the M59s in the similar S/N neighborhood.
S/N C40045:


S/N C41341:


S/N C42557:


Anyone ever seen clear signs that early C block rifles were converted to M59/66A1a like this and not reserialized?  This is a totally new one for me! 

[Loose}{Cannon]

Researching the LBs, I have seen old oddball mentions of 59s being converted to 59/66 models.  I fail to see why it would need to be reserialized.