Is the A not just a re-issue of the same catalogue item, in the same livery and same basic model, but often with a different loco number? You tend to see it more commonly with the coaches and wagons
R2413 will be a BR Blue Class 31 (in fact 31270)
R2413A is obviously 31174 as seen here ... another BR Blue Class 31
If you look at the coaches they are the same A, B, C, D, E releases all with different numbers but the same basic model and livery.
There is also an R2413B - 31268 in weathered BR Blue