It's going to be any member of the BH16/WH16 range of drives. If you're asking particular drive, there are two generations of drives thus far, but it's probably a first-gen BH16NS40.
Generation one includes drives from BH/WH14... through BH16NS48. You can crossflash from/to anything in this range, with the BH16NS40 (or BH16NS48, depending on the region) being the halo drive.
Generation two recycles gen one model names and is based around the BH16NS50/NS55 introduced in 2015. The NS50/55 is the halo drive. Don't think anyone has wanted to crossflash any of these yet.
(Note: we haven't seen a second generation BH16NS48 yet, with jadburner just the other day mentioning one with a November 2015 manufacturing date. Second generation drives had begun to be released by then.)