The older Bosch grinders were metric- I cant believe they no longer
stock the metric lock nuts for holding on a wheel.

The Bosch grinders are all set up for hubless grinding wheels- which is
not to say you cant use one with a threaded insert, but any standard
hubless grinding or sanding disc will work with one if you have a pair
of the locknuts.

If you really cant find any from Bosch, email me and I will dig around-
I think I have some for 4 1/2" Bosch Grinders, which I believe will fit.