A reader of the web site shared photos of their Schwarzer concert zither, model number 23 1/2. It has serial number 10097 on the first fret of the fretboard, which dates it to 1913. Following is the catalog description for this instrument:

No 23 1/2 - Imitation rosewood top, sides veneered with tulipwood, bottom polished black; ivory celluloid bound edges and inlaid wood purflings; pearl figures inlaid around sound-hole; neatly engraved German silver patent head; finely finished; in cloth covered wood case, flannel lined, nickel-plated corners, catches and trimmings, with lock and leather handle. $50