I have seen a couple lenses with the non-AI metering tab (the OP's "blade?") flipped around, so that it is too far back for the overhang of the F3 prism. This resulted in damage to the my own F3 (see the attached picture) one time when I did not check before I attached a "new" lens. In my case, it was possible to mount the lens but it did leave marks.
EDIT to add: Anyway, if you're using an F3, the rabbit ears aren't necessary and can be removed from the lens.