Mel - I also have the E.14 I headstamp .455 Auto with a GM bullet, but mine has an addition two broad-arrow marks, one on each side of the Mark I stamp - E.14 ^ I ^
Regarding proof loads, there is the Kynoch “Purple stripe” proof load for sure. I have one in my collection. But, I also have a “Purple Stripe” E.14 I with an identical stripe to the Kynoch round. In the case of the proof, the Eley military headstamped round has the more normal CN bullet.
There are also two bunter variant, black-case .455 autos, headstamp C. 17 I, which I believe to be proof loads for testing of the Colt M1911s purchase from Colt in caliber .455. The “C” is only found on black-case rounds that I think are proof loads, and on a dummy round with the word “DUMMY” right on the headstamp. No ordinary ball round is known. That is not surprising. There was plenty of .455 auto ball ammunition available from England to send to Colt, but there would have also been a requirement for dummy rounds and proof loads, so the lack of a ball round with “C” headstamp is not surprising to me, and I believe the “C” to stand for Colt. It has NOT been positively identified in England and in my view, while there is controversy over what the “C” stands for, for me it is very logical that it stands for “Colt.” That is reinforced by what I am told is that the “C” rounds are more commonly found in the US than anywhere else.
Regarding the Kynoch proof loads, there are actually specimens with two different extractor grooves and extractor-groove bevels. I have one of each in my collection.
Although Fiocchi cataloged this round after WWI, I had, at one time, a box and a half of the Fiocchi boxes, but both contained E.18 I - headstamped cartridges. I don’t believe that Fiocchi ever actually made this caliber, but rather had acquired, from whatever source (I have no idea where), a large enough quantity of British rounds to actually box them and sell them commercially. It is my belief that the .455 Auto Round, including the three versions earlier and different from the standard one, was only made in England and India. I don’t know how long they made them in India - my specimen, the only one I have see myself, is dated from 1925. I am sure there are many more of these specimens around. It would be interesting to learn of any other dates.
There are so few variations of this round, I would think less than 40, that I wish I had collected dates in this caliber when I started collecting, and much more of this caliber was around.