To me that looks like a fresnel lens.
The fresnel is made for the projector that was installed in your Hitachi set.
Thinking either the original RGB projector or replacement will project at the same distance is in question.
I’m looking forward to your outcome.
yes it is a fresnel lens, I'm trying to cut it and make it fit. what should I use for the cut?
I would use a straight edge, Marker and a skill saw. Than sand your edges with a fine grit Sandpaper.
it looks like you have 3/4" around the frame for give so it should be easy.
I don't know if a fresnal for your replacment projector is a good idea though. I would us RPF and keep it simple.
I used fresnals in the past to magnify screen in DIY projection and your replacement projector doesn't need help with the throw and may hinder your ability to tune it.