I have two of these CGA, both with "91 xxxxx" which is the year imported, I can't remember but CGA was a importer or something, both of mine only have the serial number on bolt and bolt carrier, the stock, magazine, trigger, receiver cover are all blank, no serial numbers, pinned barrel, almost like new, I have them stored with my other prized SKS rifles.
I forgot this: Mine have the bayonet rivet, not the screw.
EDIT: I found the following info with a google search:
CGA seems to be a mark associated with "China Guangdon", one of the hundreds of "U.S." importers owned by Chinese, most of which disappeared after the late 80s early 90s.
Maybe a SKS expert will let us know.