Sounds to me like you are correctly describing the cartridge. It should have a copper primer cup.
There is another round with the same headstamp generally referred to as the .455 W&W (Webley & Scott) Model 1910. It is basically the same cartridge as the regular .455 Webley Auto Mark I, except that it has a much thinner rim. It has a copper primer cup as well, and a full metal jacket RN bullet of the normal ogive for the .455 Webley caliber. It also has a norrower extracotr groove and extractor-groove bevel than the regular .455 Mark I.
There is a little-known round we have been calling the Model 1910/1912 Transitional, for lack of a better name. It is headstamped " • ELEY • 455 AUTO and is somewhat rarer than either the 1904 or the 1910 rounds (the dots on the headstamp are not perfectly round dots as I have had to show them). It is the same as a .455 Mark I but has the thin rim of the 1910 model. However, the extractor groove and bevel are normal for the later .455 Mark I, and it has the smaller primer of the Mark I. Primer cup is copper. Bullet is normal .455 RN FMJ CN.
Then, of course, the Webley .455 Auto, sometimes called the Model 1912 Mark I. It was made by several factories in England, and also by Kirkee, in India, the latter during the 1920s Mine is date “25” and headstamp K^F 25 I. The “^” is used here to represent a broad arrow on top of the letter I", which is the Indian version of the Broad Arrow Government Property Mark.
The stadard Mark I round was made for the military and commercially in England, and made in Ball, Proof and dummy rounds. There may be other loadings, but if so, they are extremely rare. I have heard of a blank, but have not seen one that I can recall. There is a solid steel, unheadstamped dummy, simply turn to the shape of the cartridge but with a flat base, like a rimfire. We know it is a “real” British round, and was probably either an armourer’s dummy of some sort, or a box-makers dummy. Thye are scarce, but most certainly NOT rare.
Your 1904 round is a very, very nice round to have in a collection. While not the rarest round in the auto pistol game, they are not common, and are simply an interesting round in all respects. I like this series very much, remaining among my favorites after 50 years of collecting auto pistol rounds.