I had been an ATC cadet for about a year when they started filming 633 Squadron. Along with a mate, Mike Cobb, we went to Bovingdon one sunny day in the school summer hols, and were rewarded with being able to get really close behind the camera for the filming of two scenes of dialogue involving Donald Houston and Cliff Robertson. We were also able to closely examine the Mossy which was to do the crash landing scene; even from a few feet away those bullet holes looked genuine rather than just pieces of painted plywood stuck on the fuselage.
There was a total of 12 Mossies at Bovingdon but only 4 or 5 were in flying condition; one other was just a fuselage used for cockpit scenes.