Thanks! Yeah, I think I agree. My pilot's notes for the Mk.V have very similar wording to what you say about the mixture control, but it covers Merlin 45, 45M, 46, 50, 50A, 50M, 55, and 55M, so there's very little to differentiate small differences between engines. However, my pilot's notes for the Mk.VI also mention removing the mixture control on later aircraft, and it only covers the Merlin 47. So it seems 40 series engines could have their mixture controls removed, and it is likely that the 50 series never had it at all.
BTW, does anyone know what a Merlin 50A was?