What’s even more amazing is that the ILM crew were working on other stuff also for ROTJ, that might or might not have been on the chopping block for Black Friday - eg, there was apparently an attack on a giant Imperial “communications ship” that had some form of modified Star Destroyer model, glimpsed in a couple of reused shots in the finished film.

And unlike Jerjerrod aiming the Death Star at Endor and flooding the tunnels with countermeasures, this wasn’t even in the script! So they were doing it for reasons not connected with simply making the shots the film needed for theaters.