P-ceiling vs. T-ceiling

[skystudent]

I'm new to the group. I've done some searching on this subject, but haven't found the answer. Sorry if I missed it:

The P337 is certified to 20000 and the earlier T337's to 31000. In the following aritcle,

http://www.superskyrocket.com/pages/.../history_2.htm

"The later T337 and P337 models are only certified to 20,000 feet, whereas the earlier turbo models have no such limitation, a quirk of changes in the certification standards that took place in the 1970s."

Questions:

Is the "low" ceiling for the P337 indeed a certification "quirk." Is it a real performance limitation (i.e. convergence of Vx and Vy)? Is it due to pressure-differential limitations on the fusilage?

Is it legal to fly a P337 above 20000?

If it is legal, any experiences you can relate?

What's the skinny, please?

Thanks in advance. This seems to be a knowledgable and friendly forum and a great find.