I also remember the actual practice tubes to use with these dummy bombs. They had a port cut into the side of the tube near the base and an angled plate welded in as a deflector.
This meant that the "drill" bombs dropped down the tube would self-eject and the crew did not have to uncouple the base-plate to tip the dummy out.
Thus the crew could run all their rapid fire drills properly.
The favourite toy was the sub-cal kit, universally referred to as the "mini-mortar" This was an insert system powered by compressed air and used on VERY short ranges. Basically an expensive, complex lawn-darts rig. The Mortarmen used to fabricate suitably-labeled, miniature target structures (like "Miss Nancy's Boarding House" and "Doug's Drugs", "Annie's Orphanage" etc), from cast-off cardboard from ration-pack shipping liners.
I thought Mortarmen were eccentric until I worked with Assault Pioneers!