Early in WWII there was not much call for 6Pr 7cwt HE shell, kinetic energy AP shot and upgrades were much more in demand. The Mk1 HE was in existence but nobody used it much because it was not very good, being rather small.
Later on when we got into urban warfare in Italy, then after D Day, there was a need for the infantry to have HE shell for their own anti tank guns, rather than rely on tanks who might be off doing their own thing. However, there was a 6Pr 7cwt shell production capacity shortage, but there was capacity for producing slightly modified 6Pr cartridge cases and the navy did not have a pressing need for 6Pr 10cwt HE.
So the 6Pr 7cwt Mk1M case was produced to suit the naval shell.
The original Mk 1 army shell was fitted with an external tracer, was the same full length diameter, had a driving band 0.77" wide, and was slightly humped about two thirds up from the base end. It was also 1 and a quarter inches shorter than the Mark 10.
The diameters of both Mk 7 and 10 shell were reduced, below the driving band by 0.022", which led to the corresponding increase in the internal diameter of the Mk 1M cartridge case. The Mk 10 was fitted with an external tracer, but the Mk 7 was not.
The driving bands of both these marks were 0.812" wide, but the Mk 10 was plain, whereas the Mk 7 had two rebates. Both were flatter in appearance than the Mk 1.
These two complete rounds were referred to as hybrids.
The Mk 1 case was also used with APCBC and APDS shot.
Plug FH 1.6" No 1 was used, but not often as the rounds were held fuzed.