We have had this plough discussion many times over the years on both sides of the pond.
My own personal conclusion which is open for further discussion if any one can clarify further.
If there is NO <M-H> on any of the parts, other than the "P" part number I don't believe it is a M-H plough.
This same question often crops up in Canada with the sulky ploughs, there are many examples out there with just the "P" part numbers, they look identical to the same plough which is stamped <M-H>, Possibly the same foundry produced parts for several different manufacturers.
It would be great if someone could shed further light onto this subject.