Here is a brass Poth Beer Tap Marker from the Poth Brewing Company located in Philadelphia, PA. This Poth Beer Tap Marker would have been fastened to a generic tap knob or handle and used to identify what kind of beer was on tap being the bar in bars and taverns. This Poth Beer Tap Marker is pre-prohibition and would have been made prior to the creation of tap knobs and tap handles. Most likely made as one off the earlier tap markers before the standardized ball tap handles that came about in the 1930’s.
Another collector who I know told me he saw one of these markers attached to a cane so there may have been multiple uses for these plaques by the brewery.