There's a few explanations for why they made them SA only, one I heard was that the British didn't want their troops trying to make aimed shots at targets beyond pistol range.
I think there were two or three models of grips that had disks, some of the unmarked ones are probably pistols that were issued to outfits that marked the frames of their pistols rather than the disks, but others could be pistols that originally had grips without a disk, but at some point the grips were replaced.