James Wily patented his inovations to the gas cannon in 1915.
These cannons were first advertised in 1916. (click image for full ad)
There were 5 models in the lineup: 16F, 11F, 7F, 11D and 7D. All five models come up from time to time on ebay. The next most difficult thing to find is the little, seperate ammo chest that came with the 7D.
The 1916 models were only made for one year. In 1917 the design was changed. The new models used cast iron drags with integral axles and larger wheels.
Ammo chests were secured with screws on the 7F and with split eared rivits on the 16F and 11F.
The 16F and 11F used a "V" bottom ammo chest.
All three field models used pressed steel drags.
The thin steel drags and the screw axles did not provide enough friction to keep the barrel at the desired elevation. In 1917 the iron drag system solved this problem.