I have a 1392464 that's missing the original F5 keycap. It's in great shape otherwise with only a few broken rivets. Bad to have a missing keycap on a board that's characterized by its unique keycaps. Other than the keycaps, mine is a typical 1987 SSK. Since everyone seems to be missing a different keycap, we need to locate a junked 1392464 for parts.
ClickyKeyboards says this board is rare. However, if you look at the IBM parts suppliers, many of them list this model.