My thought is if these 300 guns were used for such purposes as guards in the San Francisco area (maybe some at Ft. Mason), many would have shown up in the US after the war. They have not.
One scenario might be that with the American fliers leaving the Flying Tigers and rejoining the Army Air Corp, these 300 guns could have been sent to Chiang Kai-shek for use by the remnants of his flight crews.
Of course, nearly all of the 300 guns could have been lost when a transport ship was sunk in the Pacific.
At this point, there seems to be no information as to what happened to these 300 guns.

Edit note: These 300 guns were shipped in October of 1942.