Just guessing here, but I'm thinking that since Empire
mods are set up for 901 attys that the drip tip was probably sized with that in mind. 510s and 901s are close enough that most drip tips interchange. The 901 is just a hair wider though. If you were to stick a 901 cart into a 510, you'd see that its a really tight fit.
That said, 510 attys also have a miniscule amount of variation between manufacturers, but generally not enough to be noticeable. The plating on the tip may also be aggravating issues. You could try sanding down the drip tip or widening the Atty, but either option will probably be more bother than its worth.
Only other ideas i can think of is that it's an 801 driptip, a eGo type A or B tip, or a 306. The 306 is unlikely and easily identified. The 801 would be slightly larger than the 510/901. The eGo tips are way larger and would be really noticeable.