Have been searching through period bicycle publications from Smithsonian Online Library:
https://library.si.edu/books-online/subjects/bicycles for info about Mason & Mason. Found a lot and still digging. Here is an overview of what have found so far.
John Mason was an active bicycle racer in Chicago area in around 1890-92.
George Mason was involved with the Illinois Cycling Club during same time.
1893-95: Mason & Mason was a downtown Chicago dealer/retailer selling Columbia, Thistle, Stearns, Gendron, Waverley, St. Nicholas, America and Western Wheel Works small line bicycles.
1894: They began manufacturing their own bicycle. Made between 50-200 in 1894. A small business compared to Western Wheel Works who made 40,000 bicycles in 1894.
1895: First time found them using the name Soudan for their bicycle. It had diamond-shaped fork crown. Produced 300 for 1895.
1896: Now make three grades of their own bicycle. Soudan ($100 with diamond fork crown), Nile ($75 with circle fork crown) and Pyramid ($65). Projected to make 600 for 1896.
1897: Same three bicycles as 1896. Projected to make 19,000.
August 1897: Mason & Mason has financial troubles. Under funded. Assets sold.
November 1897: A new company called Soudan Manufacturing Co. opens in former Mason & Mason factory in Chicago. Managed by John and George Mason.
1898: Soudan Manufacturing made only one model (diamond and drop frame) and only sold to jobbers.
1899: Still only making one model for jobbers. Projected to make 35,000 bicycles.
Fall 1899: Moved out of Chicago to a new factory in Elkhart, Indiana. Did not sell out to American Bicycle Company Trust. Remained an independent manufacturer.
1900: Soudan Manufacturing offered three models and the optional "Nile" circle fork crown on all 1900 models. Crown looks same as the one they used on 1896-97 Nile bicycle.
February 1901: Soudan Manufacturing closes.
May 1901: Elkhart factory sold.
Did not find many images of Soudan or Nile bicycles. In reference to the tandem shown at the beginning of this thread, the circle fork crown definitely looks like Nile. From what I can conclude is that the circle fork crown was used 1896-97 and 1900. Nile tandems were only made 1896-97.
1893:
1894:
1895:
1896:
1897:
1898:
1899:
1900: