If the VIN indicates it's a 1500, 2 door, which it does - it's a conversion, almost surely done by Centurion as they were the "biggest" company doing that particular job, though they didn't do many. Some detective work around the frame and floor will reveal the chopping. Vehicles change hands enough times the seller may not have any idea who did the work, just that he knows it's uncommon.
Edit - after a quick re-read I guess I get your point, though you could not buy that truck configuration from GM. I know most of us here know that. You had to get a 2500/3500 and accept the longbed (or wait until the very last years where you could score the rare-ish 2500/3500 crew shortbeds) and heavier duty truck with all that comes with it.
I'll admit when I first found out about them I thought they were pretty cool but the more we learn about build quality, I would pass on them. Same with the 3500HD crew cabs; apparently done very poorly. The 3500HD 4x4 conversions also done by those same companies also sound like hack jobs.
Richard