That got me poking around in my shop for other examples I knew I had. Below, a set of "flat" version wrenches:
They also offered a style of "raised panel" wrenches (raised on the one side, but recessed on the reverse):
What's interesting about this style is that it almost exactly matches a larger wrench I have that is marked "GMC" (presumably for General Motors Canada):
I also have some Canadian wrenches marked "Durable":
These have recessed panels on both sides. Note the maple leaf:
As does one style of Gray wrench, with virtually the same font of capital letters used on the Durable wrench:
And which also includes the maple leaf on the reverse side:
Below, the Durable wrenches and the Gray wrench combined:
Note that the three-digit catalogue numbers line up, from the Gray wrench on the bottom and through the Durable wrenches above:
I think this is pretty strong prima facie evidence that Gray made Durable.
This was no small toolmaker: the Durable line also included sockets, adjustable wrenches, and pliers.
Now look at the catalogue numbers for the DublHeX wrenches:
In this series, the three digit catalogue numbers are identical to the Gray/Durable ones, except that the first digit is a one rather than a zero for each fractional size. (E.g., the 3/8"-7/16" DublHeX 180 wrench corresponds to the same fractional size Gray 080 wrench.) Coincidence? I think not. Given that Gray also made wrenches under the "Dreadnaught" name, I believe that Alex Gray must have had a predilection for brand names beginning with the letter "D." I submit my case.
Update: A friend recently gave me a copy of the 1953/1954 catalogue for the J.H. Ashdown Hardware Co., Ltd. of Winnipeg, Manitoba. They were distributors for a wide variety of products retailed by hardware stores. Below, their offerings for both Dubl-HeX and Durable. The latter ad proves conclusively that Durable, at least, was a Gray product: