OM-1's (chrome) were produced with prism foam up to approximate serial serial number of about 1,110,000. Above that number (and continuing to where the 1N chrome started) they were produced without prism foam. MANY old OM-1 bodies show deteriorated prism foam by dark green or black blotches in the viewfinder. Once a blotch shows up, it means the old foam has turned to goo and eaten through the paint and aluminum coating on the prism. The only rela fix is to replace the prism. OM-1 Black were also made with prism foam up to the same serial number. Unfortunately, ALL 1N bodies were made WITH prism foam. Hope this helps!
Originally Posted by BruceN