The lead character in the serial number is actually a "U". The format belongs to United Engineering Corporation, a Taiwanese company and known contract manufacturer of Diamondback. Your frame was manufactured in February 1990, which corroborates it being a 1990 model.

I was unable to find the 1990 Buyer's Guide. I must have been thinking of another year. However, I was able to find an advertisement for the 1990 Apex , Ascent and Ascent EX. The Ascent is shown in your colour scheme and both the Apex and Ascent appear to match your frame design. I wouldn't be surprised if both models used the same frame. The Ascent EX frame shown is a slightly different design. It has a straight brake bridge, no rack eyelets on the upper seat stays and appears to have horizontal, or at least different, rear dropouts.

Edit: I believe the 25.4 x 30.0 dimension should refer to the noiminal quill diameter of the stem to be used with the fork and the inner diameter of the crown race to used with the fork. Basically, they're the compatibility dimensions. The thread should be 1-1/8" x 26TPI.