I too found sand brutal and we only used glass beads.
I too had one, 328219 and it came with the round pin in the buttstock and a flat pin inside. Without knowing why the black, nice one wouldn't be inside I installed it, in 1976 or so. Then later I changed it out so it wouldn't be worn like the flat one. Again without knowing. I shot it while installed and had no trouble, but trouble was documented and I'd have wept to have destroyed a rifle like the pics of the one in Pyle's book...since the rifle is sold and last I knew it was in a collection of WW2 M1time capsules in London On. 5 rifles all original and with the 5 different inspector cartouches on the stock. Top one in the pic was mine...