Glad someone remembered the diverter valve. It works together with the handle valve to actually control the flow.
As far as receipts for warranties: may not be necessary. The usual procedure is for the parts distributor to do a direct exchange. Grohe didn't even ask me for initials, but Delta used to ask for paperwork. Contact head office to find out the nearest warehouse-distributor location. Do not fall for the 'mail it in' suggestion unless your are really in a remote location or can do without for about two weeks.