I am making a collection of songs and ballads using sexual euphemisms, as expressed in previous threads, with a view to an anthology when I can get round to typing them all up. I have about 30 that include millers. I'll have a quick look through to see if these others shed any light on this one. Most of them just use the grinding metaphor when the girl comes to get her corn ground and of course the 2 stones. I wonder if the cliche of stones for testicles in such as the Derby Ram actually evolved from the milling metaphor.