Perhaps the double-rim was inserted into a ferrule/opening at a 'feed' (maw) location for the raw grain to be 'metered' through before grinding , as a funnel (wide portion goes up) .....
Having seen only a couple of old stone gristmills (waterpowered) in operation , I will comment that they seemed to grind rather slowly .... seems like they all had good-sized feed-hoppers/bins , not small like your cool little feller , TNVFL3 .
I'm in the dark on this one ... sorry for just guessin' !