In article <46qb7f$sh@elna.ethz.ch>, nh@lem.ee.ethz.ch
says...
(all deleted, some correct, some not)

Re: Differences between Nikon FM, FM-2, FM-2n ---
FM --- shutterspeeds 1>1/1000 sec., 1/125 synch., noninterchangeable vf screen, liftable AI tab to allow use of non-AI lenses with
stop-down metering, meter on-off switch in the advance lever.
FM-2 - shutterspeeds 1>1/4000 sec., 1/200 synch. (no metering at 1/200),
interchangeable vf screens.
FM-2n- same as above, but 1/250 synch. (so there is metering at the top
synch speed).
None of the FM's has TTL flash metering (alas!), all have DOF preview,
mechanically-timed shutters, 3-LED meter, AI lens hook-up.
Hope this helps.