jabberwoki,
That's a nice find - and a mighty fine gift for a friend!
In case you didn't realize it, W535-AC-26577 is a WWII US Army Air Corps contract number. Plomb had seven (7) total contracts with the Air Corps between 1941 and 1945. I will look it up for you later tonight and you can pass the information (exact dates and total contract value) on to your friend if you want. Historical touch points like that add to the allure of a NOS box, in my opinion.
I will second Jim's opinion that it most likely contains WF- tools. I've had a few of them over the years, gone now in trades. (I've never seen one with the "Pl
umb" mistake by their cardboard box supplier, though! Given the infamous long-standing duel between the two company founders and the eventual resolution, that mistake makes your box even more entertaining, special and valuable!)
Based on the length of the box, my guess is handles of some kind, perhaps WF-9 sliding tees. Sockets came in dozens but in a box that was more square.
See thumbnails for some photos of a lot of US Army Air Corps 9/32-inch drive stuff I picked up awhile back, from mixed mfgrs (Williams, Snap-On, Hinsdale), including a bunch of sealed NOS Plomb boxes. The one depicted had WF-11 (1/4" service opening) sockets in it as I recall. They're neat to have as conversation pieces among collectors, but, ya know, what are you really going to do with a dozen sockets all the same size except admire them and the history attached to them?

I suppose one could sell them one at a time to guys that need them to fill holes, but that always seemed distasteful to me, personally.