Hello fellow Northfieldian! If the choice is between 1980 and '90, then it's a '90 (they weren't being made in 1980). Luckily the catalogs are online.

https://www.sheldonbrown.com/bridges...ne-1990-18.htm

1990


But of course if the serial points to being made late in 1990, it could be a bike for the next model year. Luckily- if you can see the RB-2 lettering, the graphics are different enough to distinguish between the two.

1991


It's hard to see if the dropouts on the 90 are vertical because of the black frame, but they definitely are on the '91. They're both lugged. Hope this helps!