I'm leaning toward this, at the moment.
At first glance the Earth Spacedock looks like a Star Trek version of the Death Star and the chasm of Excelsior like the hangar bay of an imperial Star Destroyer.
But the romantic in me says that this is just a deceiving appearance while instead the ILM designers and model makers possibly approached their Star Trek III creations rather with the intention to feature these as “counterparts” to their “cousins” in the SW Universe.
...one design purpose of the Excelsior’s hangar bay could have been to enable the “repair” of smaller vessels since it appears to have essential characteristics of a miniature onboard spacedock to substitute the lack of such facilities at the outermost borders of Federation space (“Star Destroyer” vs. “Star Explorer”).
Interesting notions there. This could help explain why the "southern" bay was so different indeed. Not just a support bay for herself, but for other small ships too. I kinda like it.
I’m definitely not suggesting that transwarp was a failure (just because Scotty had sabotaged the T-drive before it even got a chance to be tested) – and in the early TNG phase Andrew Probert considered the Enterprise-D to use transwarp drive where merely the “trans” had been dropped – but the modular nature of the components inside the chasm, from ST VI on, is rather evident, IMHO.
Interesting. I didn't know Mr. Probert regarded it that way. I always tried to mitigate the notion that it "failed" by saying that the result was a conventional system that was still light-years better (no pun intended) than conventional warp drive. I'm in the minority, but I've always interpreted transwarp to be along the lines of a jump drive. Styles' lines in TSFS might seem to agree with this. Then again, the line "All speeds available through transwarp drive..." is a tantalizing one.
(And, I concluded transwarp to be a failure by the fact that they didn't call it transwarp on TNG, and later by the fact that other races had something called transwarp.)
These modules look like they might perfectly fit into other Federation vessels, but stick out like a sore thumb inside the chasms of the Excelsior Class: A module designed exclusively for the Excelsior Class would fill the entire available chasm space, but what we see leaves plenty of unused (and wasted) space port and starboard of these modules which begs for explanation.
So in other words, you're contending that the module is the transwarp drive system, which was in fact successful and later revised for other ships into something less... modular? But for the Excelsior class, they had to remain with this odd semi-vestigial modular weirdness?
In general I feel that the cargo transport capabilities of other starships next to the Enterprise (i.e. Miranda and Excelsior Class) still merit further evaluation.
Agreed.
Where was Excelsior assembled? Given the apparent shortness of time between ST II and III obviously not in the orbital San Francisco dockyard because this one had been occupied by the Enterprise at the beginning of ST II (and seems to small to accomodate Excelsior).
Why can't there be other drydocks? (This is something I've always assumed to be the case.) In Generations, we see an apparently larger one inside of which the Enterprise-B was built. Perhaps it's the same one in which the Excelsior was built?
Can we exclude the possibility it was assembled inside Earth Spacedock? Our protagonists seem rather surprised to see her there at the beginning of ST III, as if they had been totally kept in the dark that Excelsior was soon to be ready (plot hole?). Or was that the "public restricted" section, accessible only to Excelsior personnel (e.g. Janice Rand) which exclusively witnessed the return of the battle-damaged Enterprise from the Mutara sector (another delicate topic).
Of course this may explain a few things. They assembled the ship inside Earth Spacedock but totally forgot that it wouldn't fit through the spacedoors.![]()
I don't think we can rule that out... but somehow Spacedock has always seemed unlikely as a starship construction facility to me, given what we saw inside. Still, I don't think it can be ruled out. And, I've never considered the possibility that Rand was on Excelsior, especially given we see her in San Francisco in the next film. I always assumed she was assigned to Earth. Then again, in TSFS she appears to be a Commander, so she might have a twin sister.

Thank you for providing that sir. I might have to take some very rapid screen caps and alter the levels similarly to see if any more light can be shed. It appears to me as if there is more of a reinforcement around the pod in this version - which should come as no surprise since I believe this model was made from the Jein version, which did have a slightly different structure inside there. I'll dig up pics later.
Also, as to R2-D2 having black panels, it's the same reason as why the Imperial officers sometimes have black rank insignia instead of blue: the blue color gets chemically stripped from the film stock when they used blue screen to optically composite images. In fact, that's why blue is the color for bluescreen; blue was the top layer of film color, so most easily removed, also, there is very little blue in a human face, so it's the easiest color to work around. But little blue panels like on R2 and those rank pips, become transparent on that film layer and the black space background shows through. If you read George Lucas's novel of Star Wars you may notice that Luke's X-Wing was originally supposed to be Blue 5 rather than Red 5. But the compositing process using bluescreen would have not worked with blue striping on the model, so they changed it to Red.
I imagine this is why the Excelsior was photographed to appear as gray as possible also, for fear of the blue parts turning transparent (and therefore black) in the final print.
--Alex
Fascinating explanation... thanks for that, too.