rx heater

Can anyone tell me how to remove RX inserts from an M Frame Heater lens without breaking the surrounding lens, and how to Insert other RX inserts?

I bought a Pro M frame and some RX Heater lenses in 1997, then shortly after, ordered another RX Heater lens. Can anyone account for the difference between overall lens shape and insert shape differences in the photo below? Can anyone give me a tutorial or any idea of changes in Pro M Heater...