The Aberdares were a long lived class of 81 engines which were built in seven lots.    

The first engine, No. 33, was a Dean design with obvious Churchward influences. It differed from all subsequent engines in its arrangement for the pony truck compensation beams. 

The next 40 engines (2621 - 2660) were built new with a parallel (DO) number 2 boiler. By 1910 all had been rebuilt with a coned number 4 boiler. The remaining 40 engines (2661-2680 and 2601-2620) were built new with the larger and heavier coned number 4 boiler. This involved a redesign of the brackets which tie the inside and outside frames together and shows in a different rivet pattern on the outside frames. 

Top feed was fitted to all the class from circa 1911 onwards and it is in this condition that the kit represents. From this kit an accurate model of any of the last 40 engines can be built from circa 1911 to 1949. There were many detail differences both between individual locomotives and as the class changed through time.

For an accurate model it is important that photographs are used and the list below details some useful sources. 
A pictorial record of Great Western Engines - Vol. 2 - OPC. 
The Locomotives of the Great Western Railway - part 7 - RCTS. 
Locomotives Illustrated number 45 - Ian Allan 
The Great Western in the West Midlands - Ian Allan 
LCGB - Ken Nunn Collection - Photomatic - HMRS collection 
Smokebox.  Originally quite short. Later front and back rings had snap head rivets.
Smokebox saddle.  Early flush rivets, later snap head rivets.
Cab spectacle windows.  Plated over during the late 1920's.
A.T.C. Equipment.  Fitted to the whole class between 6/1930 and 10/1931 

When built the locomotives were paired with standard 3000 gallon tenders which many kept until withdrawal. From 1929 onwards many were paired with 4000 gallon tenders of Great Central Railway design made redundant by the scrapping of ROD 2-8-Os.