i have a om2 (dont remember if its the 2 or 2n) and the prism was degraded due to the foam. Still usable, but noticeable black patches in the viewfinder.
Good tip: the OM10 uses the same prism. I replaced the prism in my OM2.. -Doable, but not as easy as I would like. The OM's are seriously packed tight, and the foam was a pain to thoroughly remove.
Somewhere i read that serial numbers past a certain point do not have the foam.