An image might help. Such is the way with these things, a picture shows up a lot of dust and grime that isn't really that evident in person. Note the change in colour at the two sides of the pressure plate - is this indicative of a camera that has had a lot of film through it? Anything that looks out of the ordinary?
New FM3a backs are available. Lots of rollage on the back pictured. Possible rust or corrosion around the upper rivet holes?