It's a Trainer, Field Artillery, Mk3.

A pneumatic job, used during WWII.

On top - a "pantel" or panoramic telescope, which is used by the gunner to orient the tube for direction. Various adjustments to level the device, train it left or right, and to set elevation.

And the compressed air system used for launching the projectiles (though I *really* prefer Jim B's take on it). Used on a 1000 inch range, they had special maps and firing tables, and this allowed for training most crew operations except loading, or shifting trails - the big muscle movement bits. You laid the trainer just like a real gun, computed data just like a real gun, adjusted just as a Forward Observer would, etc. But it cost a lot less.

Back when I was a Lieutenant, we used the 14.5mm trainers - small cannons firing custom ammunition, They were way cool and I want one. There are some available, but they're in Israel, dangit. I can't afford the shipping, not to mention the import license.