The following pertains to 1950 and later. The 302 era is a whole 'nother story...
The Bell System would never willingly have used non-WE parts. If you find a phone with Bell markings and other manufacturers' parts in it, that was done by an indie refurb house at some later time.
GT&E stuck almost exclusively with AE in the same manner.
Indie telcos other than GT&E would of course mix & match freely, as they could have just about anything in their inventories.
For anything from the 1950s or later where the dials had extended number rings:
Dials:
AE 80/90 and SC 1543/1553 were one cluster, all the 500 sets were another. You may find AE dials on these SC phones. You won't find SC dials on these AE phones. You will never find WE-style dials on these AE or SC phones unless a collector did it to confuse his like-minded pals:-)
Among the 500 sets, dials from any manufacturer could end up on any other manufacturer's phone. ITT and SC swaps could be fairly common.
There was a rare adaptor made that enabled putting AE or SC dials on a 500 set. Not often seen. I have one example of the adaptor.
SC made a 500-like housing to "update" the looks of the 1543. This is lower and wider at the very front than a "real" 500 housing. If you see what looks like a 500 set with an SC 1543's dial in it, look carefully at the front of the housing. Think of this as SC's version of a 5302:-)
You may find the late 70s AE dial fingerwheels on earlier AE dials. These are the fingerwheels that are one-piece plastic but mounted to the dial spindle with one screw right in the center of the plastic disc (without the 4-lug "spider" that AE used in the 60s / 70s). The number labels for those were self-adhesive. The way to tell if looking at a picture, is to look very closely for the 4 lugs of the spider behind the ribbed circle inside the fingerholes: if you don't see the spider, it's the late 70s fingerwheel.
Handsets:
Handsets could more readily be swapped across all manufacturers because they were pretty well compatible all'round. However, once again, AE tends to stick with AE, and anything 500 (G1 and G3 handsets) may be found on anything 500.
Ringers: Very rarely if ever. Transmission networks: Almost never. Hookswitches: never, ever.
Cords: There were aftermarket manufacturers so you may find cords that differ slightly. For handset cords, AE handsets will always have AE style cords, 1543 handsets will almost always have 1543 style cords, and G1/G3 handsets on 500s will always have the appropriate cords for those. Rarely you'll find a G1/G3 cord on an SC 1543 handset, but never on an AE handset unless done by an end-user (the give-away is that the grommet does not fit the hole in the handset shell).
And of course housings were interchangeable among all 500 sets so anything goes.
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To my mind the only unacceptable thing is what I call "contrary repaints," where one color of plastic is overpainted with a different color paint. When the paint wears off with use, you see the contrasting color of plastic underneath, which looks nasty to me.
Proper factory repaints in the same color as the plastic are acceptable but less desirable than unpainted plastic, because the painted surface is more difficult to keep clean without scratching.
Mixed dates are OK because these things were designed to be repaired and recycled indefinitely, so the mixed dates are almost like a statement against modern consumer obsolescence. Ma Bell was the global high point of eco-industrial design, with maximum efficiency of embodied energy & materials, so a phone with a 1950s chassis and 1970s dial and 1980s handset components was an example of how to build something the right way to absolutely minimize waste.
The term "Frankenphone" was new to me when I signed up for this forum.
I'd suggest the term "mongrel" for phones with more than one manufacturer's parts, e.g. an ITT base with an SC handset. This by way of resemblance to the fact that most Americans are "mongrels," with two or more (often four, sometimes more) nationalities/ ethnicities/ races in our ancestry. That mongrelization helped make us stronger and more vibrant as a nationality and a culture, so you can think of it analogously for phones: the ability to mongrelize was one of the great strengths of the 500 set, and contributed to its longevity.
