Whooa. You're all right but... The interior bore dimension on the black plastic bit that you screw the nib holder onto is too small. TB sell the Empress as a Spring Blossom, and if you note, David has declined to sell the fountain pen option because of this issue. It was supposed to have been resolved last March but it appears that the manufacturer has shrugged off the problem. Axminster have not been too proactive about the problem and have continued to stock the pen despite the issue.
However, the International Standard Short cartridge is the most popular pen cartidge in the world and all the kits (that I am aware of) use this cartridge. The standard governs the aperture that is pierced by the pen when you push the cartridge on to the pen, there is no standard to dictate the length or the width of the plastic container itself.
The problem with this pen is that the bore is very tight and cheaper chinese cartridges that are poorly moulded don't fit or get trapped in the pen. The cartridge can get left in the pen when you unscrew the nib assembly. There are nineteen quality suppliers of international short cartridges and I think it was called Diamen that is the chosen ink of the Writing Equipment Society - yes it does exist! I spoke to their technical officer who is involved with setting European standards for things that need standards with regard to the writing industry and he confirmed the issues above regarding the apertures about a year ago.
Some of the branded short cartridges do fit, but do you trust your customers to continually use the recommended brand? Or in fact do you trust the brands not to change the source of their plastic cartridges?
The two cartridges in circulation mainly are the Parker or long cartridge and the International Short Cartridge, there is no "Standard" cartridge as such. There are one or two others but this issue was caused by Parker trying to monopolise the ink industry. I had loads of problems with customers complaining they coundn't get the quality cartridges, ignoring the advice and getting ink all over the place when they got stuck, mess, dont make em any more, and they didnt sell very well when I did!