During the 1st Session of the 75th Congress, the Pennsylvania State Commission sponsored an amendment to its original Battle of Gettysburg 75th Anniversary Commemorative Coin Act; the amendment was introduced in the Senate by Senator Joseph Finch Guffey (D-PA) in April 1937.
The new proposal looked to amend the 1936 Battle of Gettysburg Commemorative Coin Act by striking out "at a mint of the United States to be designated" from its first section. Such a change would have effectively erased the one-Mint restriction included in the original and opened the coin up to being struck at all Mint facilities of the time (i.e., Philadelphia + Denver and San Francisco).
When the original coin bill was introduced in 1936, it did not include a Mint restriction; the sponsor was looking for coins to be struck at all Mint facilities. Before receiving approval by Congress, however, it was amended (by the Senate) to striking coins at just one Mint facility - later designated by the Mint to be the Philadelphia Mint facility. From the start, Paul L. Roy, the Executive Secretary of the Pennsylvania State Commission, desired a P-D-S set of Gettysburg coins, so the addition of a restriction to available Mint facilities was a major disappointment.
The 1937 amendment was a last attempt by Roy to secure coins from all three US Mints. The bill was referred to the Senate Committee on Banking and Currency, where it languished and was never reported out. Roy's hopes went unfulfilled once again!
Having coins struck at each of the three then-operating Mint facilities likely would have increased overall sales for the coin. Though 50,000 coins were authorized, sales of the "P" (Philadelphia) coins amounted to a disappointing 26,900; the balance of 23,100 coins was returned to the Mint to be melted.
Beyond the Senate's intention of protecting collectors, striking the coins at the Mint in Philadelphia created a historical connection. Both cities are located in Pennsylvania - about 140 miles apart - and Philadelphia produced the bulk of the nation's coinage in 1863 - the year the Battle of Gettysburg was fought; the San Francisco Branch Mint produced some, the Denver Branch Mint did not yet exist (it opened in 1897).
1936 Battle of Gettysburg 75th Anniversary Half Dollar

To read about Executive Secretary Roy's "Three-Mint" desires, check out:
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1936 Battle of Gettysburg - Ephemera IIFor other of my posts about the Gettysburg half dollar, see:
Commems Collection.