I have always thought that the J61 was seen as a different aircraft to the ATP with a different type certificate,
Nope, listed in my license as "BAe ATP/J61". Hence my question as to how much change (if any) there was between the two? I was told on the type course at Woodford that the two "are different, we put together a differences course but because they never sold (any J61s) we binned it"