The original bill calling for a commemorative half dollar to mark the 100th anniversary of the admission of Arkansas to the Union as the 25th State, was introduced in the 73rd Congress (Senate) in February 1934 and became Public Law on May 14, 1934.
In the 74th Congress, additional coinage bills were introduced in an attempt to expand the one-design Arkansas program to include three new designs. The three-design effort failed, but one additional design was eventually approved on June 26, 1936 and the Arkansas-Robinson coin was born! The Act enabling the new coin specified "one additional design to be placed on the reverse side of not less than twenty-five thousand and not more than fifty thousand of the 50-cent pieces..."; the Act served as an Amendment to the Arkansas coin's original legislation.
An initial order for 25,000 Arkansas-Robinson coins by the Arkansas Honorary Centennial Celebration Comission was struck in January 1937; all of the coins were eventually sold (thousands via bulk deals with coin dealers), none were returned to the Mint to be melted.
The Robinson Amendment included a specified expiration of June 26, 1937. Past this date the Commission could no longer request the Robinson coins, even though it still had 25,000 coins legally available to it. With the expiration of legal authority drawing near, a bill to amend the original Arkansas Statehood Centennial Coin Act to allow for between 25,000 and 75,000 Robinson coins to be struck (without an expiration date) was introduced. This new bill would have supplanted the coinage limits installed with the 1936 Amendment Act and allowed the Commission to order/issue coins from all three Mint facilities (i.e., create P-D-S sets), and issue the coins over multiple years with multiple dates (as it was doing with the original Arkasas Statehood half dollars).
The US commemorative coin market had softened significantly by 1937, and the move was likely an effort to reinvigorate sales by creating additional coins that collectors would "need" to keep their collections comlete.
(Note: the last issue of Arkansas Statehood Centennial half dollars (Original design) took place in 1939.)The proposed amendment was introduced in the Senate and referred to the Committee on Banking and Currency. It did not get reported out of Committee for further consideraion, however, and thus, collectors had to
make do with the available 1936-dated Robinson coins - based on reported, individual collrctor sales, there were plenty of the coins to go around!
1936 Arkansas Statehood Centennial - Joseph T. Robinson Half Dollar

To learn more about Senator Robinson and this coin, check out:
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1936 Arkansas Statehood Centennial - Robinson.
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1936 Arkansas Statehood Centennial - Robinson in Coins with Public Service Theme ThreadYou can access other of my posts about the Arkansas Statehood Centennial coins and various commemorative coins and medals here:
Commems Collection.