It's not a stupid question! Our 320s have 4 different series tail numbers. When started we started with N503JB and moved upwards, 504, 505 and so on up to 599. So the next series is the 600 series N603JB and on up. The 600 series of tails have the rounded silver overhead bin latches and had different seat backs for the TVs but the 500 tails have been updated with the new seats but the 500 tails have the small gray square overhead bin latch. The 700 tails start at N703JB and those have the newest interior, the larger overheads, and the touch screen Flight Attendant Panel in the front galley. The 800 series tails are the newest 320s we have on property and most of those planes have the winglets, sorry sharklets . The 900 series of tails are our 321s.

It's basically just referring to the N registration, NxxxJB, on the aft part of the aircraft. Each series of registrations have subtle differences from the others.