Another 35mm print of The Great Mouse Detective showed up on eBay. This one is incomplete, but it's interesting because...well, look at the sample frame.

In recent years, there has been a degree of snobbery about how certain animated titles should have been open-matte (Transformers: The Movie, The Secret of NIMH, etc. - more amusing if you remember that a few years prior people were complaining that the original DVDs were full-frame!)

But this is proof that for animation releases, 4:3 does not always mean open-matte. If it were truly open-matte, you would see stuff like this (and in fact, some 4:3 releases do have this problem, can't remember which ones though). The films were composed and protected for widescreen (even if some of them look unnaturally tight at that ratio - not Disney, but An American Tail is a great example. It's sometimes too cramped at 1.85, but quite often too loose at 1.33...)