Yes that pesky little cap that is always coming loose and going missing.
Can anyone with an FM or variant and one of these other cameras verify that the caps are the same? Assuming of course that you have a cap!
Digital SLRs - D200, D1 / D1h / D1x, D2 / D2H / D2Hs / D2X, FUJI S3 S5 Pro
Film SLRs - F5, F100, F90 / F90x, N90 / N90x