Wico Type A Magneto Question

I am looking for a replacement magneto for my project. The model currently in use is a Wico A 839 Z. I have identified a possible replacement but its type number is prefixed with an X as in A X839 Z. Does anyone know the significance of the X prefix. I'm sure I've read something about this before but cannot recall what it was. I thought it might have been a reverse rotation indicator but the two examples in question, the one with and the one without the X prefix, seem to be the same rotation based on the position of the impulse trip mechanism.




The original magneto spec plate with type number A 839 Z, with impulse trip on R.H.S.




The possible replacement unit marked type A X839 Z, also with impulse trip on R.H.S.

Any advice on the meaning of the X prefix and the usefulness of the possible replacement magneto welcomed.

Neil.