Here's what I've figured out so far about Planet of the Apes:

The original movie was mixed in 4-track stereo and mono, though there is some doubt as to whether the 4-track mix ever saw the light of day (there's a discussion on that subject here: http://www.filmscoremonthly.com/board/posts.cfm?threadID=71763&forumID=1&archive=0) - at least the majority of 1968 release prints were mono.

By the time home video rolled around, Chace did one of their pseudo-stereo mix jobs and this is used on all the laserdiscs that I'm aware of (which may not be exhaustive, but I've spent some time searching). DVD and Blu-ray brought questionable 5.1 mixes to the table, BUT the Blu-rays do feature the original mono mix as well so this is the best source that I'm aware of.

It's a similar story for the sequels, though in their cases there was no question of an original stereo mix since they were only ever released in mono. They too follow the path of dubious home video remixes followed by a solid preservation of the original mono on the Blu-ray.

I'm prepared to be corrected on any/all of this, particularly if anyone has more info on the original theatrical releases or laserdiscs that preserve the mono tracks as PCM - this would be especially welcome in comparison to the BDs' basic Dolby Digital.