.303 Adaptor cartridge?

Can anybody please tell me what this rather unusual cartridge is? It is very similar in dimension to the .303 Wilkinson Adaptor having a caselength of 21mm. It has virtually no rim at all but has a small square projection to the side of the head which presumably aligns it for some reason when chambered.

Your cartridge is a Kelly-Andrews .303 adaptor cartridge, patented about 1900. The cartridge slid up into the adaptor case, with the little projection fitting in a groove on the inside wall of the case. The firing pin mechanism then slid up into the adaptor case against the head of the cartridge.

Thank you Guy.