Sam, I did a bit more digging and found some compelling evidence for Plomb as well as P&C. As was mentioned before, they were already part of the same organization, so it's really not that hard to believe. At least that gives you options.
Anyway, I started with this period pic of a heavy wrecker. Dated April '43 which shows the truck at Camp Gruber, Oklahoma. Identified as an M1 wrecker so, a bit earlier than yours, right?
The toolbox appears to have the same construction as the box upthread. There is a large hasp coming down the center/front and there is a seam through the middle of the toolbox where it folds open. Fairly distinctive design.
Not the best pic, but I think you can see what I'm talking about.
Then a bit more googling, led me to a picture of a Plomb box that sure looks military to me! Actually owned by another member of this forum.
It's really hard to find a good high res (period) image of one of these wreckers. But I looked at quite a few. Most that I found appeared to have a different type of box mounted to them. The 'bubble' on the front of these boxes is also a distinctive feature, but I haven't yet nailed down who made that box.
Here's a good image of the box I'm talking about.
That box is on the back of this wrecker:
So, it looks like there is at least one other option for you. But you said you already have the box anyway, so now you just need to fill it up!
Happy hunting! And be sure to come back here post pics of some of tools as you acquire them!