Back in the late 1970s I heard a Europe-bound C-141 call London "with ya at 410" - I was quite astonished that the old banger was as high as that and can't recall ever hearing anybody else above 39000 when having an airband radio on as much as possible was my thing (usually 134.9). Subsequently flew over to Miami on an AA 767 which was mostly at 41000 according to the Captain's announcement...