Going back to the strap in the original post, I picked up a couple more recently, one still has a label, these are certainly pretty similar to the 42 inch later tool kit web straps but they are still the wrong length and I can't help but think the "MC" is a red herring.
I think you're right Rob. I asked on another group and there were a number of examples used on artillery pieces etc...no possible connection to motorcycles, and for wartime use, there only seems to have been the standard 42".
The label refers to webbing type C and multi-purpose begins with an 'M'...