Seems a lot of you are familiar with these things. Any idea how to determine the approximate year this thing would have been made or sold?
I don't know enough about these to be of much use but I can say for certain that it was made before the name change from "Plomb" to "Proto" in 1948, by a third party manufacturer who was under contract with the Pendleton Tool Company. I believe that these were made for a number of years, if not decades (1930s and 40s), without any identifiable marks, like date stamps, to determine exactly when they were made.
I'm certain others here can offer more useful information but if you're wondering, these boxes are highly sought after, but only by a handful of collectors. Most people don't know who or what Plomb is and seeing an old, worn-out tool chest, throw them out or sell them for next to nothing at yard/garage/estate sales.
Brian