Gilles
Clive showed me that photo at work the other day and asked what it was for, so either it was you that sent it to him or the photo is making the rounds. There is a fellow in West of me who says he has used these in the reserves sometime in the late 50s.
Here is the similar setup on a Bofors.
https://www.youtube.com/watch?v=qy_fWmKMSSA
Sub cal has been around a long time. It both allows cheaper ammunition, as well as allowing "live firing" on much smaller ranges than would otherwise be possible.
I have a bracket out in the shed that I suspect is to hold a rifle on an artillery barrel for a similar sub cal device. There are also similar (but much smaller) brackets to attach a service rifle to the side of a Boys rifle to allow indoor training.
The Canadian Forces is presently in the market for a 14.5mm sub cal trainer for the artillery. Now the battery can just load their guns in a panel van and head to the range.
http://www.casr.ca/doc-npp-artillery-trainer.htm