Hi, It was in order "The BSA Air Rifle"...Basically a BSA marked Lincoln, They faffed around with the taps on some of these, next up was the Improved Model B, same sights as before, but tap had retaining plate and was set in a socket and had retaining plate, as opposed to the through tap on the earlier gun.
There was never an ''A'' or "C"...Next was the Improved Model D, which has beefier sights, (the aperture on the stock is an add-on on this example) and an improved spring loaded plunger set behind the loading tap to give a more positive tap closure (previous ones just shut to a stop).
These are the main visible differences, Airgunners then were just as keen on the latest "best thing" and BSA announced each small improvement with a big hoo-ha and advertising campaign which increased sales dramatically.
They started fitting a side latch cocking lever in the later stages of production of the "D's" and then did away with the steel butt plate...these late pre WW1 guns were the first "standard" air rifles, a title that all sizes of 1919-39 inherited (apart from the break action).
ATB, Ed