I've made a list of the Vosper MTBs that were fitted with captured Italian 20mm Breda guns at some point in their service history. No doubt there were more, these are the boats known to me :
MTB 61, MTB 77, MTB 82, MTB 85, MTB 86, MTB 89, MTB 97, MTB 226, MTB 243, MTB 290 and MTB 298.
MTB 61 (a 70 ft Vosper) is particularly interesting because it was turned into a gun boat by adding three Breda guns (source Coastal Forces Veterans forum) :
1/42 8th MTB Flotilla
(allocated for service in Mediterranean)
MTB 73 at Greenock
MTB 75, MTB 76, MTB 77, MTB 78, MTB 79, MTB 80, MTB 81, MTB 82 to join on completion
8th MTB Flotilla
7th MTB Flotilla
25/01/43 MTBs 61, 77 and 82 arrive in Malta.
15/2/43 MTB 61, MTB 77 and MTB 82, depart Malta for a patrol off the island of Marittimo (South West Sicily). MTB 307 and MTB 315 are to rendezvous in the patrol area. MTB 61 has been converted into an MGB by landing the torpedo tubes and fitting three captured Italian Breda 20mm guns. MTB 61 suffers a failure of the hydraulic steering gear and goes to tiller steering. "
It would be interesting to have a pic of this particular conversion. I wonder if this IWM pic could possibly show the vessel in question. The front section of the torpedo tubes appears to have been removed and I assume that another Breda gun was mounted on its port side :
WITH THE MTB'S ON PATROL IN THE MEDITERRANEAN. MARCH 1943.. © IWM (A 17655) IWM Non Commercial License
This pic probably shows the same vessel and a Breda gun on the port side :
WITH THE MTB'S ON PATROL IN THE MEDITERRANEAN. MARCH 1943.. © IWM (A 17656) IWM Non Commercial License