18 prefix = 18th year of Chinese sks production or 1973.
From page 1 of the thread:
(This list is by no means complete; more will be added as they are discovered.)
1967: Triangle Arsenals, S/N in the 1,200,000’s.
1968: Oval/Triangle Arsenals, S/N in the 1,300,000’s.
1969: Triangle Arsenals, S/N in the 1,400,000’s.
1970: Triangle Arsenals, S/N in the 1,500,000’s. Note that there is a distinct but small (less than 20k rifles) group of /26\ rifles produced with a 1.5 million prefix. The Chinese must have known they would be tapering SKS production at /26\ in 1970 and modified the S/N accordingly.
1971: Triangle Arsenals, S/N in the 1,600,000’s.
1972: Triangle Arsenals, S/N in the 1,700,000’s.
1973: Triangle Arsenals, S/N in the 1,800,000’s.
1975: Rectangle Arsenals, S/N in the 2,000,000’s.
1976: Rectangle/Triangle Arsenals, S/N in the 2,100,000’s.
1977: Rectangle Arsenals, S/N in the 2,200,000’s.
1978: Rectangle/Triangle Arsenals, S/N in the 2,300,000’s.
1979: Rectangle Arsenals, S/N in the 2,400,000’s.
1980: Oval Arsenals, S/N in the 2,500,000’s.