I would have to agree that the set-used mp-40 canvas pouch was almost certainly an original. In 1976 they were insanely cheap and easy to get. The WWII reenactor and collector markets which exploded in the 90's did away with all of that. The problem you are having with finding the perfect match to the on-screen version is
#1) there were numerous manufacturers for the Wehrmacht making these things all through the war with production processes and values constantly degrading until the end in 1945. This leads to a WIDE variety of all sorts of manufacturing differences just coming out of the factories (if you are wondering why some have the D-ring in the corner as opposed to coming from the side... this is the pre-war style, largely made for the MP-38). So even if you score an original, chances are you would have to alter it (please don't do this!)
#2) there is not one modern replica maker that makes a perfect replica of any of these vast varieties of war-time pouches. There is always some little detail that is wrong. For instance, the pic of the set you have as 'high-end' at a whopping $228 (way too much for any replica imho) has side stamping on the leather straps. They didn't do this during WWII. This is a post war thing only. It ain't correct.
Additionally, the real pouches were actually used in a war. I suspect the long strap on the reloader pouch (the small pouch held the mag reloading tool) of the sceen-used one was quite possibly a field repair by a soldier who actually used these pouches and entrusted his life to their proper function (the MP40 was notoriously unreliable in cold weather). Field repairs are very common features on these things. Or maybe it was an 'on-set repair.' Either one of those scenarios are more likely than a scratch built ammo pouch replica.
I think the best you can do is get one of your 'middle ground' sets and make alterations to suit the needs of on-screen accuracy. The military tour ones are quite good for replicas. You can also try hessenantique.com. I do not recommend the IMA version. They are not very accurate. And stay away from that czech stuff on ebay. Bad, bad, bad.
Oh, by the way, there was no special tan version of the MP-40 ammo pouch used in Africa like you see for sale these days. This is a fantasy item. There were indeed special pouches made for 'tropical use' which saw action in Africa, but these were made in olive green canvas and had straps made of webbing, not leather. And they certainly were not tan canvas. All types were used in the North Africa Campaign, not just the tropical type. Since the Axis occupied Tunisia until the last of the DAK surrendered there in 1943, it is possible the prop crew could've found Mp40 pouches floating around on location to use in addition to what they brought, although, I would agree, probably not likely for the prop -- though it is not entirely implausible. I have a friend who spent some time in Tunisia back in the 70's and he said there was still a crap load of Axis hardware still sitting around back then.