The fourth stamp in this series is the 50 Mon. As with the 25 Mon stamp, this green stamp was never issued and therefore any stamp claiming to be used has a fake cancellation. The stamp is designated Scott #4 and Minkus #N4. The stamps were produced in 1884. Black and white images are from Brady-Tyler and colour images are from MikeG Canada (Stampboards) and other sources.
Above: block of four genuine stamps.
The Catalog Value as shown by Stampworld is 12 Euro for Mint. No Watermark and perforated 8½ to 10.