One clue is the idle speed stop screw which is located on the left side of the carb rack in the OP’s first pic. This would suggest that the carbs came off a 1971-74 750 K which had petcock on the right side. The 1975-76 750 K bikes had the petcock on the left side and the idle speed stop screw on the right side. Honda didn’t place the idle screw and petcock on the same side from the factory.