Bearing Blocks

What problems could I come across if I was to use a BK12 bearing block instead of a BK15.
The reason I ask is that the BK12 mounting holes just about fit on the slots of the 45x90 aluminium profile. If it was only the centre height of the bearing then a spacer could be used.
Am I missing something obvious?

Bk15 blocks tend to be fitted to 20mm screws and BK12 to 16mm but there's really no reason why you couldn't use them.! . . . It's just not standard procedure so you'll need to specify BK12 machining if your ordering 20mm screws other wise you'll get BK15 machining.!!