Generally old playfield plastics from before and including the 1940's, 1950's, and many years in 1960's. Should not be flattened by heating them, they will become more brittle and break into pieces. This info is on Pinside some where.

For the playfield plastics, RyanClaytor has done a few reproduction. The EM Topic is titled, "RyanClaytor Plastics Available from PBR".

For other plastics there is replacing them with new parts, if new ones are available. Then the new plastics will not be the same as the older ones. As they age, there is a marbling in the plastic that becomes more noticeable. Thing like, Ultra Violet exposure, solvents, solvents in older style playfield polishes, have done their damage. Lets not forget heat damage from the G.I. lamps.

I do remember something about old plastics from those time periods decomposing(?) over time, Shrinking, color change, And even putting off some kind of fume(?) or something that will corrode metal parts. Pocket and Wrist watch Plastic Crystals from that time period are a prime example of this! Not sure if the same type of plastics were used in pinball or not. Just saw the thread and kind of remembered something about it from working on watches as a hobby in the past.

I would imagine that it applies to most plastic of the time? And UV and heat from the lights, mentioned already.... Maybe it all plays into what happened. Even on my couple of projects from the 80's and 90's , the plastics are brittle and break pretty easy.

Looking at your photo, with out reading your comments. I would guess that that game sat in an open structure. Rusted metal, even under the playfield glass, heat warped plastics, do not think that a small bulb can do that. Is that pin from a more southern climate, near an ocean? Hopefully Pinball Resource has the correct new pop bumper parts.

Old plastic from the 1940's & 1950's wasn't anywhere near the same as the plastics we know today. Most contained animal fat and other stuff not associated with modern plastic and most early plastics did odd things as it aged. I think it is more uncommon to find early machines with decent plastic parts than it is to find stuff with lots of issues. Or at least that has been my experience with earlier plastic stuff (pinball related or just other antiques with plastic parts).

Cellulose Nitrate & Cellulose Acetate are two of many types of early plastic. You would be amazed at how quick some of that stuff goes up in flames if exposed to an open flame!

All I remember is something about the reaction going on in the plastic and age has some kind of reaction with metal. Or will corrode metal.
Too many pins might have the direction with that on the Nitrate and Acetate. I have no clue... I'll try to find an article I read on it. And post a link here if I find it. Basically it decomposes or starts reacting or something. I just know an old plastic crystal on a pocket watch will destroy and corrode the metal in the watch from whatever happens to the plastic. It shrinks, starts turning color, etc.....

ok..I'm just rambling...I'll go try to find the article...

Here's a link that gives a really good description of what happens. This isn't the one I was looking for but will explain some until I find the other one. And I have no clue what type of plastic was used in pinballs. But the descriptions of what happens seem to match your pictures.

https://www.nps.gov/museum/publications/conserveogram/08-04.pdf

I have a Mr and Mrs pacman. I finally got it working with a ton of help from an awesome member from pinside. Months of him helping me in PM's and threads. All the plastic drop targets looked great!! I took the drop targets apart, cleaned, touched up the paint on them, made them look almost new!!! After all of this? I broke most of them in one or two games from them being so brittle.