M4 finder in M2 body, possible?

is it possible to fit M4 finder ina M2 body? and is M4 finder a much better finder than M2's,

in terms of coating and brightness etc?

"Is it possible to fit M4 finder in a M2 body?" - Yes.

 

"And is M4 finder a much better finder than M2's, in terms of coating and brightness etc?" - No. Essentially, the only difference between them is the M4 finder has 135mm frame lines added.

BTW: Early M3/2 finders were put together using Canadian balsam, which often deteriorates with age and ends up turning a dingy yellow. Also, the older finders are prone to separation, which looks like spots in the finder. Late in the M3/2 run (about 1.1 million and above in about 1964) production of M cameras used better cements. If you have an M2 finder that does not show this separation, IMO after this much time it is unlikely it will deteriorate in the future.

 

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Yes it is possible and has been done. Leica USA replaced the range and view finder in my M2 with a rebuilt M4 finder (becuase no rebuilt M2 finders were available) when I sent it in four years ago for a CLA and finder repair. My M2 is a fairly early button rewind.

Some M2s have the funky depth of field notches.