From what I've read, the earlier P47-M's didn't have the fillet. I, too, have only seen a few examples. The fillets were apparently sent out later on, for field mod. (BTW, the purpose of fillets was to utilize the spiraling slipstream coming from the prop and to provide for a little more directional stability)
Incidentally, the 1/32 kit I am building now has an M prop and the fillet. (Just not the right magneto, accessory drive for the P-47M engine, for those of us who split hairs over such things) It's the P-47 kit with Gabreski's plane on the box.
Another interesting note about the M, is that many of the guys who flew them, in the 56thFG, were Jug holdouts. They basically refused to allow themselves to go to P-51's. Also, in the 56th, the jugs were painted with whatever they had in stock or could "find". The brits had a lot more paint stock, so the blue's, greens, etc. are british based.