Here is some information about the history of BSA Bantams. 

Bantam

Originally this was a copy of the DKW RT125, then developed further by BSA.

D1  1948-63, 125cc, 2 stroke single, 50mph!!

Single saddle, rigid rear and telescopic forks. This had a plunger rear end on later models. It was a unit single engine with a 3 speed gear box.

D3  1958-61, 150cc, 2 stroke single, 60mph!!!

Rare machine this, being a bored out version of the 125cc with improved front forks and brake. The later model has a swing arm frame. It also had an improved lubrication system, which means it is less prone to seizures.

D5/D7/D10  1958-68, 175cc, 2 stroke single, 65/70mph !!

The D7 had the benefit of a better frame, forks and guards. The engines a bored out version of the previous model. The D10 had a relatively modern chassis with a much more improved bakes and suspension.

D14/175  1968-72, 175cc, 2 stroke single, 75mph!!!

The D14/4 was the best Bantam. The D14/4 had a 4 speed gear box. Most of the problems had by now been ironed out, this is a nice bike with better handling and reliability.