if it was a FE or other electronic shutter one then it's a flat battery... but for a FM, no idea... I have read where the foam around the mirror box (not FM or even Nikon specific BTW) can get sticky and catch mirrors... never seen it so maybe that could be something to investigate. I just went and had a look at my 20yo FE and it's foam is decidedly claggy although the mirror isn't sticking yet! My newish FM2n has nice foam.