That could be a tricky one. Without an actual photo of the MTB it could be one of several possibilities.
1) A 20mm Oerlikon mount ( either a single or double manual mounting or a twin power mount)
2) A single 2Pdr Pom-Pom.
3) A 6Pdr in a power mounting.
4) A short barrelled 4.5 inch in a power mount.
If you can't find a pic of the actual boat, see if you can find a boat near to her pennant number. Boats built in series tend to have similar fittings.
Of course you need to consult "Allied Coastal Forces of WWII, Vol 1" by John Lambert and Al Ross. This will give you drawings of all the above mentioned gun mounts.
The pivot question will be answered by the detail drawings but usually the heavier mounts stood on a fixed pedestal so pivoted under the footplate.
p.s. It might help to know
Wartime Activities;
Part of the 64th MTB Flotilla with MTB 724, MTB 725, MTB 728, MTB 738, MTB 739, MTB 742 and MTB 749. Based in Portsmouth
Took part in Minelaying for Operation Neptune - Invasion of Normandy
Post war, was sent for target towing and sunk as a target in 1946.