You can admire this magnificent Seidel & Naumann in the Velorama museum in Nijmegen, Netherlands. Serial number is 15815, which leads me to think it was built around 1890.
In the 1892 Seidel & Naumann catalogue they advertise that they built 20.000 bikes in the first four years, this is the period 1887-1891.
The bike has been completely (re) nickled, which was a popular extra in this top-level market. Look at the original pedals!