Some of the early Mk1M guns still had the second dovetail. I have such a gun in my collection (dewat unfortunately). It would seem some of the mk1 features were still to be found during the changeover to Mk1(m).
One could also remove the dial type sight as the dovetail was the same. This would not leave you an alternative if your scope (or indirect fire sight) got broken however.
Edited to add that I see I spent too long looking for my photo and Kev beat me to the punch.