The A class could be considered an enlarged S class 0-6-0T, and the S class a smaller R class 0-6-2T. The As had a smaller boiler than the Ms and Rs.They had 4ft 4½in driving wheels like the S class, the Rs having 4ft 6in. Stephenson’s delivered 10 As in 1910, and Hudswell Clarke 6 more in 1911.

The A1s were essentially the same as the As except that the boiler was pitched higher, had a Belpaire firebox and was fitted with top feed. There were also small differences to the frames. Three came from Hudswell Clarke in 1914, two more in 1916, and three from Stephenson’s in 1918.

Several of the A class were given A1 boilers, which put them in the A1 class. By the time of the grouping the A class was reduced to ten and the A1 class increased to fourteen. They were allocated numbers 52-75 by the GWR. The sub class complications continued under the GWR and some A1s received A boilers and vice versa as boilers were exchanged at heavy repairs.

From 1929 standard 10 boilers were introduced and these were fitted to about half the class. GWR bunkers appeared on most, and GWR cabs on some. Withdrawals started in 1948 with some of the earlier locomotives, and the majority went between 1953 and 1955. 
 

The GWR created the same boiler fitting drawing for both variations. The Std 10 boiler sketch above shows 1914 (A1) frames, but the differences are small. I also idly wonder whether the frames from the three different builders of A1s were identical. There seems little point in drawing both. As usual with the RR classes I owe a substantial debt to the Welsh Railway Research Circle's excellent publication on the Rhymney,   Welsh Railway Records Volume 1, but a GWR drawing reproduced in Russell was also very important, especially in sorting out the differences between the frame types. 

Sandboxes... You'll see the A has the front sandbox above the frame, and the A1 below. Some As definitely had the sandboxes changed to under the footplate. Another in life change was that the front suspension springs were moved to under the frames on a couple of the locomotives in the early 1950s.