From a 2010 thread.
Originally Posted by
RobbieTunes 
.There was a Diamondback Master and a Diamondback Expert offered, probably to try and keep some of the interest from the earlier Ironman Master and Ironman Expert. Some were lugged, probably only the 1990 or maybe 1991 models, most were not. It appears the DB Master and Expert were close to the Centurion Lemans and Lemans RS. I think the DB Master and Expert evolved into the Master TG and the Expert TG, which were TIG-welded. I had a nice 58cm Expert TG wtih compact triple RSX and Matrix wheels, a nice setup and pretty light, too. The tubing was simply labeledl "100% Chrome-Moly" by then. Seems the tubing decals changed every year around that time.
The Interval and Momentum were not that far apart in component levels, but the Momentum was probably lighter, as the Interval, I believe, was either HiTen tubing or something suitably in that weight range...
T-Mar....Right from the beginning, in 1990, the Diamondback Expert and Master were the TG version, using the oversize, TIG welded Tange tubes. While they were intended as replacements for the Ironman models, the components were downgraded one level to make them more competitve in the shrinking road bicycle market. To the best of my knowledge, the only steel, lugged models to bear the Diamondabck logo were the 1990 Momentum and Interval. In 1991 the Momentum would be dropped from the line and the Interval would receive the TIG welded frame with oversize tubing. Diamondback's sole steel entry in the higher end road biccle market would be the Prevail TG, a 1991 model with Shimano 600 Ultegra and Tange Prestige. In 1996 steel would give way to the aluminum Podium series.
Edit: Yours looks like a '91. It came equipped with 500 EX