Rare mandolin banjo built under patent by August Pollmann of New York. Banjo mandolins with a mandolin neck and a banjo body are common, but this is very unusual (I've only seen a couple in the last 30 years). The headstock roundel reads August Pollmann's Royal Mandoline Banjo, and various patent dates on tuners, headstock and body suggest a date in the later 1880s or possibly 1890s.
The top is of fine-grained spruce, with violin-style purfling. This insturment is probably from the middle of the original range, as the back and sides are of maple, combed with a grained finish to resemble Brazilian rosewood. (The other one I've seen was fancier, and was definitely real Braz.)
The bridge is non-original, but the tailpiece is marked Wm. Gerke, Providence, RI, and is correct for these instruments. The neck is (probably) Spanish cedar, as used by Martin on some of its plainer instruments, with a 'double cut' style headstock, an ebony nut and a Brazilian rosewood fingerboard.
This instrument is in excellent condition for its age, apart from a scrape across the back, and there is no sign of any repairs having been carried out. It is all original except for the bridge and the (modern) geared fifth peg. Straight neck, low action, no playing issues. Plays like a banjo, but with a subtle, dulcimer-like, sound. Now, when did you last hear the words 'subtle' and 'banjo' used in the same sentence?
The case is c1920s, English.