David et al
Given the clear description of cannister presented in the works of Benton and also Gibbon, i think the cannister is likely to be a dud round. If the round had fired as designed, the amount of surface contact (the very bottom end of the tin sheeting say 1-2mm thick) the cannister had with the expanding gas pressure would be minimal, so it would be unliekly to have traveled any great distance. If the base plate was in place, then their would have been a greater area in contact with the gases and the cannister, even if empty have travelled more due to great exposure to the gas expansion.
Assuming the base plate acted as some sort of plunger, if the soldered joint had failed, and pushed the balls our of the cannister given it took the main force of the expanding gas, it would be unliekly to be in place within the cannister assuming a) it had not jammed in the cannister b) all the balls had been expelled from the cannister.
I assume therefore, I may well be wrong on my hypothersis, that for the cannister to travel any distance from the gun and be observed, the base plate would need to in place or in the cannister as it provided a greater surface area for the expanding gas to act up and propel the cannister down the bore, for this to happen the cannister was likely to be a dud or only partially devoid of the balls it contained.
This is my opinion, not fact, and I may very well be wrong. My opinion is based on the amount of metal from the cannister in contact with the expanding gas through surface area.