What you saw today Steve was a very interesting piece of military history mate. The gun in question is what was known as a De Lyle carbine which were based on the Le Enfield (SMLE) action. Basically this gun was designed as an assassination weapon for use by Commandos. The gun was chambered in .45ACP and used sub sonic ammuntion. Invented for covert missions they dispatched many a German sentry.
The prototype weapon was originally built and chambered for 22lr. The second generation was chambered in 9mm but was woefully ineffective so this later saw the weapon chambered in 45 ACP.
I think there were only around 130 produced that saw active service so very very rare these days .
There was another silenced weapon of course that was extensively issued to Airmen during WW2 that being the Welrod pistol which was much more common than the De Lyle but obviously it had very limited range
ATB,
Pat
Here we go below!
https://en.wikipedia.org/wiki/De_Lisle_carbine
. A 45 ACP is not naturally subsonic 
Originally designed to launch a 230gr bullet the 45ACP has many bullet weights available. Once you go from around 200 grains and below the velocities rise. As an example a typical 185 SJHP round would probably touch between 1150 and 1200 fps depending of course on the powder of choice and available load data. A round becomes subsonic below 1050 feet per second. The 230 gr bullet was never the most accurate in 45ACP so lighter rounds were soon developed and used extensively by the military.