GWR 1854/2721 CLASS LOCOMOTIVE KIT
BRIEF HISTORICAL DETAILS
These two classes of Swindon built O-6-O tank engines were originally built with saddle tanks.
Apart from some significant mechanical differences between the two classes, which do not effect the appearance of the engines, the principal differences were: 
The 1854 had plain coupling rods and underhung springs.
The 2721 had fluted coupling rods and volute springs.
 
These saddle tanks were amongst the first to be fitted with pannier tanks, from 1909 onwards, until all (except 1879) were so fitted by 1933.  So, from this kit any of either of the classes can be built as pannier from circa 1909 to 1950. They were very long lived the majority being withdrawn after WW2 and all by 1950. 

For a detailed history of this class, including details of boiler changes and the fitting of pannier tanks, Part Five of The Locomotives of the Great Western Railway published by the RCTS is essential reading.  

The available Swindon drawings were used to design this kit but much information was unavailable and has had to be deduced from drawings of other similar classes and from photographs. G.W.Engines - Vol 1 by J.H.Russell on pages 99 - 101 and Vol 2 on page 178 has some useful photographs.
Note that fig 456 & 457 in Vol 2 are either 1854 or 2721 class and not 1813 class as captioned.
 
VARIATIONS POSSIBLE WITH THE KIT 
Pannier tanks.   Early, up to c.1917 flush riveted. From c.1917 to 1924 snap head riveted. After 1924 welded seams. 
Smokebox/tank front.   Early plain front with ringed door. Later snap head rivets. From c.1920 the smokebox had a pressed front with Churchward type door without the ring. 

Bunkers.   Early short with railed top later sheeted in. From c.1924 an enlarged type were fitted. 

Cabs.   As built were open with a canvas covered wooden roof which was later replaced with steel. A significant number were rebuilt from c.1924 onwards with new enclosed cabs some whilst carrying the old style bunker. 

Steam heating. Although essentially shunting and light freight engines many were fitted with steam heating.

Balance weights.   Changed from large type with visible rivets to a smaller plain design.