it is curved in 2 directions, and also the grooves for the brake discs go all the way through, and they would catch the stones, even if you had stones that were curved in 2 directions.
Someone else's pics from CABE ads (my bearing surfaces are fine) :
And another. The piece at the left with the serrations is dragging on the serrations inside the hub.
Maybe they all have serrations inside the hub, but if they do they aren't deep and the hub doesn't typically go RRTTRRRRTRRRRRRRRRR as one surface drags over the other on coast unless you get some exact spot between drive and brake.
Yea sounds like maybe the hub is internally damaged? I just got thru cleaning and lubing the model D in my 41Columbia and is running smooth as silk. I freaked when I had the hub apart and saw these ugly drag marks running along the center of the hub but turned out to be a thin layer of dried caked up grease that was easily scraped off.