This is my favorite crate/utility hammer.
That's a BHM. Is it marked Bridgeport on the other side or BHM somewhere on the head? Lowentraut also used the same brand name, confusingly, but I'm 99.9% positive that's a BHM.
Very few examples that I’ve seen.
Same. I have never seen one in the wild. And I don't remember seeing one on the BHM thread, either. I've seen them in the 1930 BHM catalog and in a few online farm/antique shows/auctions (e.g., Donnelly, MVWC, etc) layouts. I don't shop at places like that, mainly because it's all about the thrill of the hunt for me, and those are collectors selling to other collectors at collectors' prices, but they can be good sources of research at times. They may pop up on ebay, but I don't shop there, either, so I wouldn't know.
I have a few vintage BHM crate tools. "Tomahawks". I'm sure you've seen them around. Not uncommon. With the riveted wooden knife or perfect handle type scales handle, and the combo puller, axe, hammer, chisel/pry head. Also a couple "Box Terriers", all steel hammer, pry and puller like your Bonner. But I have not yet been fortunate enough to find a "20th Century" in the wild. Nice find!
I recommend you post that on the BHM thread. Click on the Stickie at the top of the forum for an A to Z list of threads, scroll down to Bridgeoprt and just click on the link.