I can count on one hand the number of monkey wrenches - of any brand - that I have purchased at flea markets. They're one of the tools that are so commonly seen I tend to ignore them completely and walk right past without more than a cursory glance. That doesn't mean they aren't cool to collect. Like hand planes, they kind of have their own universe. It's just not one I'm interested in diving into encyclopedically, or even enough to know a rare bird if I saw one. Something has to catch my eye as unusual for me to pick it up on a whim or instinct, and in this case, it was the ferrule.
It has the routine "COES WRENCH CO. / WORCESTER, MASS." marking (see Pic 3) on the static jaw. Rather than the more commonly seen "MFD UNDER L. COES' PATS / STEEL" marking on the flip side, however, it's marked "L. COES / PAT'D / DEC. 15 1891 / APR. 30, 1895" (see Pic 4). Those dates refer to patents 465,347 and 538,411 respectively. See Pics 5 & 6 c/o DATAMP.
Those patents were focused on improving the continuity of the bar-shank, the ferrule, and the wooden handle halves. The visible part of that design modification are those pieces of metal protruding into the wooden handle halves, which Loring Coes called, alternately, "tongues," "braces" or "arms," but most frequently "tongues." In Loring's applications, "to render the handle more firm and rigid in connection with the bar-shank," and to provide "over-locking support."
I wouldn't call them rare. This isn't some ultra valuable score. But, per Davistown Museum, Herb Page, and DATAMP, they're apparently less common.