Sks Serial numbers.

[Westcoastbro]

Im not finding any information on a BP prefix for sks rifles.
My rifle is dated 1950 it has a rebuild mark which should explain the different barrel number. But the stamped serial on the bolt cover and main upper starts with BP followed by 4 numbers. Any idea if there is documents about this BP? I cant find anything and would like to learn more about this rifle in particular

 

[Buck1950]

Search on SKSboards .com. Lotsa info there, and on Yooper john pages - https://www.yooperj.com/SKS.htm