I have just googled "Bosch X lock" and now realise that the discs are indeed proprietry. I would guess that Bosch are hoping that other manufacturers will take up the system, rather like SDS which is now a standard. There were plentiful supplies of the X lock discs available, but not much chance of getting one of the bargain grinders from Screwfix, there seems to be no stock left. Makes me suspicious that the whole X lock system is a dead duck about to become extinct, seeing the price.