I know the difference between a 510 atty and 306 atty. Both have a 510 connection, both similar at the basic stuff, coil, bridge etc. The 510 atty has a cup you can drip into and stick a drip tip on, the 306 atty was the predecessor to the 510 atty that used cartridges that slid onto the top with pre-filled foam or sponge. 901 I think is a 901 battery connection, though I could be mistaken.