As previously discussed on this site, IBM Model M (and F) keyboards have additional "secret" keys (used for alternate layouts) under certain over-sized keys.
Typically, the secret keys under the left shift key, the right shift key, the backspace, the Num-Plus, the Num-Enter, the Num-Zero, and the Enter key will produce scan codes (although, the Num Zero sometime doesn't, and the key under the Enter produces a backslash). On some keyboards the extra barrel under the space bar will produce a backspace scancode (but finding a keystem that fits this position is very tough).
What's wonderful about the additional keys is that they can be used as additional modifier keys (with a bolt-mod and some AutoHotKey / Soarer's Converter magic).
So, without further ado, behold: my new APL keyboard!
Note the poorly labeled (but very functional) "APL" modifier key next to the right shift key.