The Forward canon or gun on the Vosper 73' type II boats was a mark VII 6-pounder gun with Mullins Equipment.
I have found a few photo's of this gun fitted to a different boat courtesy of the UK's Imperial War Museum, https://www.iwm.org.uk
Any gun larger than 20mm is usually called a canon, the British 6 pounder had a bore of 57mm, the term 6 pounder comes from the weight of the projectile (6 pounds = 2.72 Kg).
Here's two of my drawings from Fusion, printed in resin! I may prefer these in brass, as the resin is a bit on the fragile side.
