The following links are to 3 photos taken at Corpus Christi in 1942 of a Navy PBY and a 30 caliber Browning AN M2 machine gun. They are posed recruiting pics which explains some of the goofiness of an aviation ordnance mate wearing a soft helmet and goggles.
Picture 1: http://www.shorpy.com/node/1001?size=_original
Note the assembly attached to the weapon over the feed and eject mechanisms. Also note that there is a circular whole visible on the left (feed) side.
Picture 2: http://www.shorpy.com/node/2555?size=_original
Feed side view of the weapon mounted on a fixed mount. Note that the circular hole is now covered by the ammunition magazine box and that a metal brace holds the ammunition magazine box in place. It appears that the metal brace is attached to the gun mount and to the weapon. The brace can be seen wrapping around the bottom of the weapon and disappearing up the right (ejector) side of the weapon.
Picture 3: http://www.shorpy.com/node/2557
The brace can be seen going up the ejector side and disappearing behind what I believe to be Russ's artifact. Note the plate on the vertical portion of the brace. My guess is that this is the Bureau of Ordnance ID plate for the brace assembly. Unfortunately, there is no top view of either the ammunition magazine box or what I believe to be an ejector box.
I just received the Base Shop Data Manual for the 30 caliber Aircraft M2 Browning. No braces/boxes are shown so we can say that whatever is on the Devastator is a Navy add on.
Without a top down picture of how the ammunition feed and ejector hardware looked, I don't think that we can go any further. I do realize that a PBY is not a TBD-1 but the time frame is the same, it's the same branch of the service, it's the same weapon, it's in a Navy plane, and it's the best we have.