There are very few parts in common between the two kits, I did originally think that it would be possible to put out one kit with alternative parts and all the Horwich bits were on the first test etch.
The differences came about as a result of BR increasing the coal space, if you look at photos of the Kitsons at Bank Hall coaled up for a day's shunting in Liverpool you will see why - coal was piled up on every possible surface, along the footplate, on the cab roof, even balanced along the handrails!
The extended bunkers meant that the tank had to be shorter so to maintain the water capacity it was made deeper. This involved a shorter chimney and the whole of the dome became visible, the steam manifold remained the same but the injectors were moved to the back of the tank. The cab was enlarged slightly - a few inches longer at the back and the frame/footplate length increased as well. Presumably for better balance a much thicker leading buffer beam was used and the cab steps were different - the only unaltered parts were the boiler, smokebox, cylinders and motion. Not bad for what was supposed to be a new build of the same design.
Incidentally the original ones are always quoted as being a standard Kitson design but i can't find anything remotely similar built by them and most of the features look very Derbyish to me.
The two test etch builds lined up on my test track, original on the left, Horwich on the right.