The MG mount and stowage went through some changes. The earlier stowage systems for the gun and mounts did not have external stowage for the barrel, and the gun and mount was stowed on the hatch ring.
A D69820 mount
This was pretty much a pain since the gun wasn't needed most of the time and having the barrel stowed across the hatch doors blocked the use of the doors. Often the units modified the stowage by moving the "H" clip for the barrel to another spot on the turret so the gun could be stowed and the hatches used.
The D69820 mount was used through out dry hull production with a lack of real stowage, i.e. up until the end of 1943. Some of the late 47 degree hull dry stowage M4A2's used the later D80030 Mount and the new external stowage in its early configuration. The Chrysler built M4's probably all came from the factory with the older set up, no external gun stowage.
Later, the D80030 Mount was introduced along with an external stowage arrangement for the gun and mount combination and the barrel (or a spare barrel). The 105mm Marc posted is a prototype 105mm tank with an experimental version of the newer stowage arrangement. The version that went into production and was used in various forms on all Shermans built from about February 1944 on, used a socket mounted to the rear of the turret with a clip that clamped around the M2 gun's barrel jacket to secure the gun and mount, weather it had a barrel installed or not, and another set of brackets the hold a barrel. The earlier production variants seen on M4A3 75(W) and M4A2 75(Dry) had the barrel bracket mounted between the two hatches on the turret roof. This is an early stowage variation as seen on a M4A3 75(W):
Later the barrel stowage was moved to the rear of the turret bustle by the addition of two short pieces of angle iron. The barrel jacket clamp to secure the gun was mounted on the left hand angle iron as well. These were used on later production M4A3 75(W) and M4 105 tanks. It was also fitted to tanks in depot or added to tanks that went through remanufacture like this M4A1:
Hope that helps,