Martin, I have to agree with 'Grouch' in that a dust extraction attachment would be almost useless, the sheer ammount of dust those things kick out overpowers all but industrial type extraction units. (I know the PWS600 is a 'mini' grinder, but mini or regular, they still produce a truck load of dust).
I've tried searching on the web, and it seems like this is just a modifier to allow you to hook your vacume cleaner up to it to suck up some of the dust as you go along, does that sound right?
If so, I've not found any actual stockists, but you contact Bosch direct:
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