Interesting bit of history. I don't feel so bad now, but I wish I would have known about the IHC receiver issue before, I wouldn't have sent it back. See below, shamelessly stolen from ihcfan55 on the CMP site:
The desire of IHC General Manager Harris to assemble the maximum number of rifles in April 1954 resulted in a curious anomaly in the IHC production effort. This anomaly is the heel void, where the receiver billet was not fully formed, but had small holes, or voids, on the surface of the billet.
When the machining was done on these billets, the machine tool cutting head was set to cut to the desired dimensions, and would not cut into the deeper heel void (since it was deeper than the outside dimensions); therefore, the heel void would be untouched by the cutting head, and would be very rough to the touch.
This anomaly is called a “natural”, and many IHC receivers with these “naturals” are found in the 4.54M-4.56M ranges. The author has a theory that these “heel void” receivers had been originally set aside and not used (due to cosmetic reasons), but were used in the April 1954 effort to maximize M1 rifle production.
But, how to smooth the very rough edges of the heel voids? The answer was deeper heel grinding. (The standard operating procedure called for very light heel grinding, to remove the sharp machined edges of the receiver.) It is theorized that IHC workers were directed to grind more deeply on the heels of the receivers to smooth out the “naturals.”
This deep grinding smoothed out the voids in the heels, and apparently made them acceptable to the Ordnance inspectors. This deep heel grinding created one more anomaly in the IHC production effort.