I always disliked the metering prisms for the F, so I never bothered with them.
I got myself an old DE-1 with a small chip out of the prism for next to nothing from dugwerks about a million years ago and have used it ever since on my F.
I'm pretty sure I have at least one FTn prism still sitting in a drawer that I'll never use, so if anyone needs one lemme know and we'll talk.