Not sure how (un)common these are, but there's one for sale on eBay right now. Looks to be in pretty decent condition too. (I'm not the seller, nor do I know the seller)

http://www.ebay.com/itm/Antique-Very-Unusual-Open-End-Wrench-Pat-Mar-15-1921-Tool-Collector-/191831564304?






Link to Datamp page: http://www.datamp.org/patents/displayPatent.php?pn=1371491&id=39645

I have the version with arms on both ends for sale on the facebook group. Priced at the same price that I got for another one I came across about a year ago. Only 2 I've ever seen out there at all the auctions and other things I've gone to.