DavidMerrick said:

It actually hadn't occurred to me that the loss of detail was due to added grain. Interesting.

If theatre prints are copies of copies, etc., what's the intermediary? Interpositive?

Here's the usual process:

Original Camera negative (OCN) -> interpositive (color correction is done here) -> internegative -> release print.

IB prints were apparently closer to the negative:

OCN -> "Matrix" 3 strip film -> release print

This is why Mike Verta is using these prints for his own restoration.