The Bristan Tap Problem

Every few years the dreaded dripping/leaking tap on my kitchen mixer tap. Sometimes I buy the washers/innards but this time I noticed I had a pair of cartridges brand new unopened with a note saying 24 teeth is correct.
I got to work removing old cartridge and replacing with new one without even checking if fit lever/tap. It does not. But rather than run onto the internet I thought why not just replace the innards of the old with the innards of the new? Which I did and so far so good.
I'd swear the old and new cartridge have 24 teeth but I think the pitch is different. Bristan is the problem. Made for Bristan must be embedded in every Plumbers Manual.