Maybe a navy medical stamp?
Here's a painting with a frame that describes a similar marking on the frame, but its hard to see if its a match. And they don't tell us anything about it (which is odd, since there's also a paper label with the emblem, which I would think might have more info). Anyway, I tried. Louis Ritter (OH, 1854-1892), "Giverny" : Lot 226