According to aerofiles, the last picture is HD.49, not 31. The difference in engine is obvious (Liberty vs Wright Whirlwind). This source https://www.deltamuseum.org/exhibits/delta-history/aircraft-by-type/crop-duster/Huff-Daland-Duster mentions that Huff-Daland tested various configurations and engines and ended with "large" Puffer (presumably Liberty-engined HD.31) and "small" Puffer (presumably Whirlwind engined HD.49) and that only the small airplane was succesfull. But they also claim that "Liberty engined Petrel 31" was the smaller machine.
To me it seemes that Delta confused both types, as the table from the L'Air Revue mentions our mysterious HD.31 as having 15.24 m/50 ft span and Liberty engine, and also mentions smaller "Petrel V" with 10,06 m/33 ft span and Wright engine. So:
HD.31 = Petrel 31 = 50 ft w/Liberty HD.49 = Petrel V = 33 ft w/Wright