Hi Mark,
It's a huge subject! Pre WW1, there were the Improved Model B and D but between the wars, the models are best identified through the serial number prefix, such as L, S and CS between 1919 and 1935 (Light, Standard and Club Standard), then A, C, T and B between 1936 and 1939. The A and T were updated L and S models, sharing the same external dimensions, whilst the C was a shorter CS (the C or Club model made its first appearance in 1930 and to confuse matters further, early short Club models were fitted with CS prefixed trigger blocks), whilst the B was a 'new' Breakdown model.
I've written a book on the subject and am presently seeking a publisher.
Kind regards,
John