What's this copper bit on my thermostatic shower cartridge for, and can I cut part of it off?
If you wouldn't take the time to return it and locate the "correct" cartridge go for it. It looks like it (the pin) keeps the cartridge centered to evenly distribute the load across the o-rings. Make sure there are no seals in the back wall of the housing that could get damaged by cutting it off and give it a try. The worst thing that can happen is that you throw it out if it doesn't work.