For 75mm I got
FM 6-70 Service of the Piece - 75mm Howitzer, Horse and Truck-drawn (1939)
FM 6-65 Service of the Piece - 75-MM Gun, M1917A1, Truck-Drawn (1939)
FM 6-55 (Service of the Piece 75-mm Gun, M2, Horse-Drawn and Truck-Drawn (1939)
FM 6-50 Service of the Piece 75-mm Gun, M1897 and M1897A4, Horse-Drawn and Truck-Drawn (1939)
any use?