^Thanks for asking him about this. At the least, we now know that it was indeed a ship, and not just a dumpster where the Spacedock crew threw their trash away
It's too bad that there weren't more ships in Spacedock when they filmed it for Trek III. They could have thrown in the other Excelsior study models, or commercially available AMT model kits. I know the only starship kits available at the time were either TOS or TMP Enterprises, but what fun they could have had kitbashing them into new designs for the background
EDIT: After seeing this pic:
I realize that there'd be no way that the study models or model kits could fit inside that spacedock model, as they'd be far too big in relation to the size of the interior. That barge model (and the Planet of the Titans study model that was also in the shot) must have been tiny indeed.