The Robinson silver commemorative half dollar is actually one of the 1935 to 1939 Arkansas issues. The obverse states "Arkansas Centennial 1836-1936" as do all Arkansas half dollars, but the obverse designed replaced the dual heads of Ms. Liberty and the Indian chief with the bust of Arkansas Senator Joseph T. Rodinson. This design was merely an attempt by the Arkansas Centennial Commission to expand its commemorative coin offering...and it's profit off of the collectors of the day. That 25,265 Robinsons were distributed at the issue price of $1.85 per coin shows there was certainly some success with this strategy.
Today, Robinsons are of course part of 144 piece complete 1892-1954 silver commemorative date, mint mark, and variety set. And Robinsons are also considered a seperate type (in a mild stretch of the concept) and so they are part of the 50 piece commemorative type set. Robinsons are somewhat more scarce than many commemoratives of the era such as the Maryland, Gettysberg, Albany, Elgin, Roanoke, etc. Robinsons are most similar in rarity to the Connecticuts. The typical Robinson has frosty white luster. Marks on the cheek of Senator Robinson are the most usual problem. But Robinsons do appear in Superb Gem MS66 condition often, and there are a good number of MS67s.