Please excuse my ignorance, but what it the meaning of this tool ?
At first I believed it was to follow the a batch of W.Nr. in a sequence of stammkennzeichen, but the disc would only need to go from 0 to 9. Also that's not make sens with Martin Handig's comment. It look like it have more to do with verbandskennzeichen (?).
Not sure I've understood. Your example works perfectly with your tool, but it would work a little worst with a know stkz batch starting with a W.Nr. such as XX42, hence a small disc [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] and a big disc [A+K+U, B+L+V, C+M+W, D+N+X, E+O+Y, F+P+Z, G+Q, H+R, I+S, J+T]. Ok that would overcharge the big disc with up to 3 letters on the same case so you can divide it in two, [X0, X1,...,Y8, Y9] and [A+U, B+V,..., S, T], the X and the Y helping you to not be lost.
Your tool works, I do not say the opposite, and if it's fine no need to change it, but last one question, why are you talking of colors ? W.Nr. and stkz are in black and no relation with the colored letter from verbandskennzeichen (as far as I know).