My cousin’s SKS Triangle 306 1.8 million serial number-range has a capital P behind the serial number. There is a space between the serial number and the P.
The whole series of number, letters, symbols goes from left to right:
Triangle 306 then three Chinese symbols, then the 1.8 million serial number then the capital P. It has all matching numbers except the mag. Any thoughts about the P!

There's all kinds of theories on Chinese serial numbers and their meanings.  But to answer your question, no, no one in the US knows for sure what it means.

CW is that the extra letter could have been added by the importer to ensure a unique serial number to comply with US import regs. It is very possible that 2 or more SKS made in the same year could have the same serial number as they were originally designated by arsenal as well as serial number, and the US only uses the serial to designate individual firearms.