Immediate giveaway that this is an M, not an F: the raised area way the top with the 24 function keys is narrow, just big enough for the keys. ALL F122 (and older M122) keyboards have this raised section extending further right, over the arrow key area, and have a square silver badge on that area.
As mentioned elsewhere in this thread, the metal back is the definitive sign of an F, but given that most eBay auctions and the like only show the front, knowing what to look for helps filter out a lot of stuff at a glance.