I can confirm that Chivenor received a permanent AR-15 installation in late 1979/early 1980, I was one of the team who put the thing in
IIRC it was on the River Taw side of the field, but it was 30 years ago so my memory may not be accurate. I have some old 110 photos somewhere, but I don't know if they will help with the exact location.
TTFN