I've noticed the small hatches for machine guns on Mk I, II, and III cabs and Male sponsons. How often were these guns actually fitted, how often were they used in battle, how were they mounted, and when they weren't, where were they stored? Philthydirtyanimal helped me a lot, but I want a second opinion, and some new questions answered. You can never be too sure, right?
In the case of the Mk I there is at least one account of the commander of D16 attempting to use the cab Hotchkiss to deal with some Germans ahead of him. Haven't got my copy of "Tanks and Trenches" handy but IIRC he had damaged the gun and couldn't fire.
The Mk III's probably never carried MG's; it's hard enough to find pictures of Mk III males with the 6 pounders in place.
Here's a Mark I with a machine gun where it should be; the picture isn't very clear (it is a still from a film) but the shadow cast by the MG can be seen clearly:
Wingate posted this very nice example of a Hotchkiss mounted in a MkI sponson. The rod attached above the barrel seems to hold the flap off the barrel, to enable sighting.