Based on the mount features described, this is most likely an Olympus OM mount, not Nikon F. The two opposing controls on the mount are characteristic of OM lenses: one is the lens release interface and the other is the aperture stop-down control. That layout is a strong visual clue for Olympus OM. If the engraving on the lens also says something like “for OM,” that further confirms it. Nikon F mounts do not use this same arrangement, so the seller’s identification was probably incorrect.