Nice find.
The thumbnail summary is that it was made by a company started by Alphonse Plomb, one of the original founders of the Plomb Tool Company, which bears his name, some time after he left the Plomb Tool Company in 1917. The company - essentially Alphonse and later his son, Henry, continued making tools marked "A. PLOMB" for decades, even after they were bought out by the California Tool Company in the mid 1920's. You may also see tools dual-marked "Calif-Tool" and A. PLOMB".
You can find much more historical info and many more examples of A. PLOMB tools, including DBE wrenches, in the 'Plomb' thread here on GJ by searching the thread on "Alphonse". You can find a link to the thread in the A-Z Index of Threads in the Sticky titled 'READ B4 POSTING!' at the top of the forum.
With a username like Plombhead, trust me when I say you will be right at home there!