Mark,
Here are some pictures of my tool pouch that belong to my 850 Mark IIA, which is the same as the 850 Mark III tool pouch.
The overall length is about 12 inches and the width is 6 inches. The pouch height is 6 1/2 inches. The top corners have a radius to them and are not square while the bottom corners are square.
The black pouch material which has leather like appearance is a non cloth backed vinyl that measures .012 inches thick.
The light grey binding has a herringbone pattern and is 1.015 inches thick and 3/4 inches wide before being folded.
The darker grey tie ribbon is a non cloth backed vinyl which also has a leather like appearance and measures .012 inches thick. It is also 3/4 inches wide and approximately 28 inches long. The ribbon is folded in half and then saddles over both sides of the top of the pouch. The binding covers this and all are sewn down together.
Pouch dimensions
Pouch back
Pouch front
I hope this is of some help, and if you need more information let me know.
Peter Firkins