Pictures might help on this one. If I understand what you are describing, I had the same challenge with my Black Knight, metal posts that are basically riveted into your plastics. I got that from the heating up comment. Once removed it is recommended that you file the metal post to level and you could then reuse the metal and just let it sit in the new plastic hole. I tried this, but in most cases I did not like the fit. I might have used a few posts this way. For the majority of those posts I replaced the metal entirely with custom cut rubber post sleeves, picture example attached.

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If its only the metal "bell" that's in the plastic, then heat up only the bell and reverse the process back into the new plastics. I did this with a very large plastic for my F2K, and it has several metal bells, I just turned the plastic art side down and heated up the bell and pushed it back into the new plastic that was flat on a table.

Heat up the inside of the top of the bell (narrow side) inserting the hot soldering iron from the bottom (wide side). Do this by touching one spot for three seconds, then keep moving around the inside of the bell, maybe two laps around. Then place the bell on a flat surface and push the plastic down firmly. It should slowly slide in. If it doesn’t heat the bell some more.

Before doing all this you should consider using the peroxide in a bag method of cleaning old yellow plastics

I have a slightly different question: I have new plastics but I am not sure how to get these front-facing ones installed since the originals are riveted into metal L brackets. Looks like I will need find a source for new rivets for the new plastics. Has anyone ever done this, or do you have an idea of how to change these out?

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Can you by the metal "bell" posts anywhere?

You can but at 4 euro's a piece....hmmm. Check out the market place for them

Just use old ones from any clear plastics