Mk. XX vs Mk. 20

[CJE]

Well, Graham, it may not be when it was built but when it was ordered, since it was at that time that the AM applied a new Mark number if relevant.

[Graham Boak]

Yes, that was my original point. However, was the precise development of the later marks of Halifax accurately predicted before 1943? The Mk.III was on the way by then, and future development depended entirely on its success. It is probably that the Mk.IV was already seen as a non-starter, but that could have lead to an entirely different development history for the type. It is fairly certain that the Mk.VI with its superior engines had been foreseen. However, the Mk.VII was a Mk.VI but with reversion to the earlier engines due to a shortage of the better ones, something unlikely to have been preplanned in 1942. The C. Mk. VIII and postwar A. Mk.IX were later still.

I agree it is very important to appreciate how the decisions to place orders for one aircraft/development or another are made well in advance of their appearance, especially when major changes/improvements are foreseen. In these specific circumstances should we go back over two years for the allocation of mark numbers to what were fairly minor variants of a then unproven development? Perhaps so..... in this case I think not, but two years is perhaps not so long after all.

[VoyTech]

Was there a different policy for mark numbers with regard to aircraft and engines? Merlin 45 entered service in Spitfire V in 1941. Some documents called it Merlin XLV, but the Arab numeral was certainly used by the end of 1941.