Probably few do. About fifteen years ago I was a controller at Chicago
Center working traffic in the Madison, WI, area. A NASA ER-1 departed MSN
requesting an unrestricted climb to FL610. I coordinated with the high
altitude sector and issued the climb. He went up like a rocket! The Mode C
readout couldn't keep up with him. Shortly after he reported out of FL600
and cancelled IFR, the Mode C read FL600. I asked a Data Systems Specialist
about it and he told me that FL600 and higher will read out as FL600.

The pilot mentions FL60 in the audio clip, also mention of a supersonic run in
Pecos MOA though, I am not sure how high Pecos MOA goes up to, but I doubt its
that high. the FL60 comment is strange too.

I would not be suprised if it was someone pulling a prank on Mr Douglas
though.
Ron
Pilot/Wildland Firefighter