Western States Importers was the company that imported bikes under the Centurion lable, switching over to Diamondback with some Centurion decals about 1990, pretty much tossing their lot to the mtb craze at the time and leaving behind the high quality Tange road bikes due to many reasons, not just the mtb boom.
During the transition, a lot of Diamondbacks were labeled both with Diamondback on the down tube and Centurion on the top tube, and some, with Avenir near the rear, I guess in a nod to the saddle, maybe in exchange for something, a la NASCAR, etc.
Most of the D'back Centurions were set up with lower level components more akin to the Lemans models of the Centurion era, which had LX100 and Light Action stuff. The presence of 105 indicates it was a higher level than the vast majority, which came with Exage and RX100, etc. I have seen one that had RSX 7-sp STI's, but only one. You'd think by 1994 that if they didn't have STI, they'd at least have 8-sp.
I can't tell due to the picture quality, but I've not seen any 90's era Diamondbacks with lugged frames, what I've seen were TIG welded, and I've not seen a Tange decal on one, but you may have the one that says "100% Chrome-Moly" on it.
That is a nice looking one, and it looks like a 56 or 58cm, maybe a Tange Levin CD headset or a dark 105. The geometry sure looks a lot like the Lemans RS geometry, a little more relaxed than the Ironman.
I don't know that much more about them. I've seen a few, but I'd have to see them together to be able to do any more comparisons or make guesses. Nice bike. I like it. I'm jealous, or course, of any Centurion I want but can't have....
Better pics would be great, and maybe hit up the Centurion serial number thread with the number.
Live your life like Coleman did.
1985 Raleigh Racing USA - Competition
1984/85/86? Klein Performance
1988 Centurion Dave Scott Ironman Master
1989 Centurion Dave Scott Ironman Expert
1989 Centurion Carbon R