I don’t see the use of the term “Operating Differential Subsidy”
Here is The State of the U.S. Flag Maritime Industry from MARAD in Jan of this year.
The Maritime Security Program (MSP) subsidy program helps offset the costs of operating under the U.S. flag. The Maritime Security Act of 1996 (as amended) authorizes direct annual stipends for up to 60 active, commercially viable, militarily useful, privately-owned U.S.-flag vessels and crews operating in U.S. international trades, in return for the owner/operators’ agreement to make the vessels available to the Government in times of war or national emergency.