I have also seen a fair number of those "E" bolts on my survivor 340 engine. In fact if you shop for "correct" engine bolts (coil, intake manfld, water pump, exhaust manifolds, carb to intake, etc.) you'll find that nearly ALL of them are incorrect when compared to all of the known original examples I have found.

Ken, do you know if your phosphate bolt was the front or rear mount bolt?

Also, I have seen a BIG variable in engine details from orange painted (Windsor) to the blue engines (Mound Road) the two engines were definitely detailed differently and likely used at least some different fastener vendors