The triangle with the chinese figures is a code.
The O223 in the triangle should be like this yes?
Thats the factory code for where it was created.
The three chinese figures you see means type 56 carbine. Should look like
If they are a different chinese letters you got quite a rare code on the SKS there.
The 21 is probrobly part of the serial number so its 21027XXXX indicating a 1977 year of manufacturing. I doubt the 21 is seperate from the way it looks, but Chinese SKS numbering systems are odd so its possible they could have done a 6 number system and beginning with a 0 would indicate a 1968 creation. I would go with the later number.
It doesn't look like the import mark included a date of import which is awkward because that would be proof enough of its preban status. As is, without being able to find the paperwork related to this import and see when that range of serial numbers came into the US theres no real way to proof its legal to add a bayonet back.