Most cartridge compatibility problems are solved by knowing one number: 510. You'd think that's the whole story, but it's not. The thread is just the start—fit is about diameter, length, and how the connection sits. If you're shopping for a gift and ignore these, you'll hand over a paperweight.

Let me back that up with what I've seen in the shop. People come in with cartridges that won't screw on, batteries that only fire when the cart is half-in, or pens that look ridiculous because the cartridge is twice as long as the battery. It's always the same root cause: they didn't check the full fit, just the thread.