The newer 24B3ST drives seem to come with firmware 1.05 on board and are reported to be iHASx24 C clones. It is impossible to crossflash a C series drive to a B series drive. If you want a B series drive, then you'll need a 24B3ST drive with firmware 1.04 on board.
If you end up with a C series drive, you won't even be able to dump the firmware from it, nor crossflash it to its iHASx24 C equivalent, since its been reported that the C0deKing's tools do not support the latest C series.
While the odd retailer out there might still have some old 'B' stock around, there's no way of knowing which one you are going to receive. The firmware revision is not indicated on the box label nor on the label on the drive itself.
The only hint might be the manufacturing date of the drive which is indicated on the label on the drive itself (the retailer will have to open the box to check it out), but since we don't know exactly when ASUS transitioned to the C series, that leaves just one reliable way of identifying the drive for sure: pugging it on a computer and checking the firmware revision.... I guess not many retailers would be willing to go to such lengths to confirm which series they stock...
If you want to make sure you end up with a B series drive, then I'm afraid you'll have to buy an iHASx24 drive which is specifically advertised as a B drive, anything else is a gamble.