Tom and Mauser&9mm have it for the loading sequence - loaded in the straight case, then shoulder & neck formed then bullet seated.
In the .303, the original ctg. had 70gr. of black powder that was in a compressed plug, inserted into the straigth case, then a wad placed on top, then shoulder formed, then bullet seated. The wad was the same size as the inner case diameter and the constriction of the shoulder above, held it in place over the powder. This load produced some 1,900fps with a 215gr. bullet and was considered exceptionally fast for the day due to normal much lower speeds when heavy-for-diameter bullets are used.

I think the early cordite loads merely duplicated the black powder load velocity.