The CACRU (Canadian Armoured Corps Reinforcement Unit) ordered a few 17-pounders for training. A 17-pounder is a 17-pounder, right? Well, it turns out, not so much.
The tank guns aren't really the same as the towed guns. But maybe they could still be used for training...
Or not. The MkI guns received and MkIV guns in use by the Sherman Vc tanks (the tanks the gunners would be using) are different enough for the training to not be applicable.
At the very least, we get a detailed breakdown of the differences between the two versions of the gun.
Seems like a pretty convincing argument. However, war is war, and you have to do with what you have, rather than what you want.
Department of National Defense RG 24 C-2 vol 12260