This is one of those "I nver noticed" it before things. Does a Nikon FM use foam light leals? Mine has none, and I've used the camera in bright daylight before with no problems. Now that I've noticed, it's going to bother me until I find out for certain.
My FM2 uses foam light seals at the hinge and along the top and bottom of the door. I think you can get by without the top and bottom strips, but if the piece at the hinge is missing you'll know it.