It's weirdness but... we refer to standard drip tips (DT) as "510" just like the mount/connection at the other end of the atty. That at least has a mechanical explanation (size and angle/
throw of the
threads), I don't know if anything similar applies to the DTs.
But it's not
entirely standardized. Some DT wells may be small and a fat stemmed DT won't fit. The opposite happens too of course and it'll be too loose.
I have one of the oldest models of iSub tanks (G) and it's just fine with all of my normally dimensioned 510 DTs. In fact, I like it's DT so much I use it on all manner of other tanks/RDAs. If there's a serious discrepancy with a new tanks' DT sizing it usually gets mentioned and I haven't heard any of that about any of the iSub family.
Bottom line... shop for any "510 drip tip" for your iSubs. Chances are very good it'll fit.