FM2N Mirror box query

Hi all, hoping one of you good people might be able to assist me with understanding the following issue. I was loaned a Nikon FM2N by a friend and it appears in beautiful condition, hardly a mark on it. It had been in storage for decades and my friend thinks maybe her drybox wasnt working properly. Anyway, initially all was normal but in the last couple of days two vertical lines have appeared in the viewfinder that I'm sure werent there before. I removed the focus screen and it looks like there somehow etched into the mirror in top of the mirror boc, though its posisible its a reflection from somewhere. Gentle cleaning with lens fluid didnt yeild any improvement. Any ideas on what is causing this? I dont want to make it worse so for now I'll hold off using it. See attached image.

Sounds like a typical Nikon situation where the pentaprism de-silvers at the top crest. I'd ignore it or find another pentaprism.

Typical Nikon prism de-silvering due to a chemical reaction between the prism and the foam between the prism and the top cover.
Three choices:
1) Ignore it, it doesn't show up in the photos taken with the camera.
2) Have the camera repaired, with a replacement prism. Good luck with that.
3) Find a (hopefully working) camera that doesn't have the de-silvering problem.

F4U said:

Sounds like a typical Nikon situation where the pentaprism de-silvers at the top crest. I'd ignore it or find another pentaprism.

Click to expand...

Thanks F4U, that makes sense, i'll just ignore it.

AnselMortensen said:

Typical Nikon prism de-silvering due to a chemical reaction between the prism and the foam between the prism and the top cover.
Three choices:
1) Ignore it, it doesn't show up in the photos taken with the camera.
2) Have the camera repaired, with a replacement prism. Good luck with that.
3) Find a (hopefully working) camera that doesn't have the de-silvering problem.

Click to expand...

Thanks, your option 1 works for me. Good to know the cause.