Gentlemen, I believe I may be able to make your day. Using the info provided, I looked through the Navy's Hospital Corps Handbook and found the answer to your question. The bag is a "Case, Canvas, Carrying" (Naval Supply Number 14-035). It's dimensions are listed as being 26x13x8 and it holds the Unit 6 or "Pack, Covers, Combat" and weighed in at 46 lbs when fully loaded. There would be 3 in the inventory for a BAS and 3 in the inventory for a Regimental Aid Station. The contents are as follows:
13-130 Pad, Heat, Chemical......4
13-135 Pad, Heat, Chemical, Refill for......4
14-250 Blanket, Combat, Canvas......15
14-280 Case, Canvas, Carrying......1
From my point of view, the 14-280 pouch (which is the common little bag marked USN with a red cross that ATF and WW2 Imp sells) is what holds the heat pads and the large bag in the photo is used to carry that and the canvas blankets.
The Unit 7 uses the same pack and is the "Pack, Combat Dressings" and it weighs in at 33 lbs when fully loaded. It carries 2 tins of combat dressings and one 14-280 pouch.
Here's a photo of a Corpsman on Iwo carrying a similar looking bag that I always wondered what it was. It's either this bag, or a Jungle Pack (speculating its a jungle pack due to the shovel being mounted on it.)http://sphotos.xx.fbcdn.net/hphotos-ash ... 6109_n.jpg