Will this set cater for keyboards with even the least common layout? I.E kbt pure + fc660M?
Good to go with FC660M but not with the spacebar, that's Leopold "unique" spacebar with awkward stems on it.
There are a few things regarding the FC660M.
1) The space bar won't fit because of the stems.
2) No dedicated arrow keys in this set. There are many blank 1x1 keys which may work for arrow keys, but I am not sure which row profile they are, so they may not be the same profile as the keys next to it.
3) Same as 2) applies to the insert key.
So other than the space bar, you should be able to get full coverage (just maybe not with the correct row profile).