Dalek
I was on 48 Hercs at Changi 67-9 and squadron aircraft went to Tan Son Nhut 2 or 3 times a week with Red Cross supplies,beer for the embassy etc. I do recall knowing that on at least one occasion an Andover of 52 had done strip landings 'up-country'.
We had a young captain on 48 (now ACM Sir JC retired) who told me he had put 'exchange tour with the USAF on operational duties in SE Asia' as his first choice in the posting column on his 1369.It was common knowledge that at that time RAF transport aircrew on exchange with MAC were not permitted into South Vietnam.
I recall meeting the pilot of the Air Attache's Devon ( Sqn Ldr Hampson or something similar) and his Dutch (IIRC) wife, when they came out to meet our aircraft.