For a detailed history of this long lived class, Part Nine of 'The Locomotives of the Great Western Railway' published by the RCTS. is essential reading. From this kit any of the 28XX class (2800 - 2883) with the full cone D4 boiler can be built to represent the class from circa 1907 to the present day. 

As the half cone, D2, boiler is not included in the kit the earliest authentic model that can be constructed is one of Lot 160 that was built with the full cone D4 boiler (Nos. 2822/23/24/25). All the variations of the many detailed below are possible using the components in the kit.

Chimneys.  Two different types are provided. 

Smokebox.  Originally quite short.  Later front and back rings had snap head rivets.
Rear sandboxes.  Originally filled from inside cab but from circa 1915 a new pattern was fited which filled from the outside.  

Front sandboxes.  Lot 210 had a larger pattern with entirely different operating rods.
Cab roof.  Initially a canvas covered wood affair, later changed to steel with two designs of rainstrip.

Cab spectacle windows.  Plated over during the late 1920's. 

ATC equipment. Fitted to the entire class between 6/30 and 10/31. 

Buffers.  Originally Dean taper type were fitted, later replaced with the Collett types. 

Brake hangers.  Originally double type later cast design. 

Crosshead. The two different designs are provided. 

Extra boiler support.  Fitted to Nos. 2800 - 2830.

Lamp bracket.  Later moved to smokebox door.

New cylinders.  From 1934 onwards, with outside steampipes and retro-fitting curved drop plates to some of the early engines.
Vacuum pipe.  Originally tall, later a shorter pattern was introduced. 

Injectors.  Originally between frames with pipes to topfeed largely obscured.  Later moved outside the frames and pipes rerouted along edge of valence. 

Lubricators. Journal lubricators fitted from circa 1920 onwards. 

Pony truck spring housing.  Originally uncovered but subsequently fitted with a bell shaped cover. 

Front struts.  Fitted from 1908 onwards.
The early engines were paired with 3000 gallon tenders of standard design.  Starting with Lot 181 in 1911, 3500 gallon tenders were used and this tender subsequently became general for the class although some engines kept 3000 gallon tenders until after the Great War.