Navy Arms.. was an early importer of a large variety of them.. I've seen ads from the '87-'89 time frame.. DIG/C&A in Va Beach was another rather large importer from around the same time frame.
Looking for information will give you a million different outcomes pending who is trying to pimp their variant.. Reprints of ads and such from the era would be a better indications......but again, not everything from yesteryear is digitized and online. Start with earliest importers and facilities in the early '80s and move forward through the cease of Chinese imports.
I will offer up, writing the current Navy Arms will do no good, been there and done that. Their former New Jersey operation is totally gone..as is all information contained within. Here is their email response to a query on SKS import information..
Mon, Dec 31, 2018, 8:35 AM
With regret we don't have the original offices of Navy Arms information, such as log books, Import records, serial numbers. This office is a
second generation on Navy Arms not selling the same items the New Jersey offices imported. Therefore, we are unable to assist.
Navy Arms LLC