06/30/2015. Remarks by Johan Visschedijk: "Three experimental O-7 aircraft (serials 25-405 to 25-407) produced under the original contract for 75 aircraft awarded Douglas after the success of the O-2 in the 1924 McCook Field competition. The O-7s had airframes identical to those of the O-2s but were powered by a direct-drive 510 hp Packard 1A-1500 engine driving a two-blade wooden propeller. Externally, these aircraft could be distinguished from the O-2s as they had only three exhaust stubs on each side of the cowling. Converted back to O-2 (25-406 and 25-407) and O-2C (25-405) configurations."