BEP has posted May print runs and new $20 2017A series P is listed. Could this be 50 sheet subject notes?
It's possible but it is considered to be unlikely. You can read about it in some detail on this WheresGeorge forum post. Being that the posted reports go from January to May with nothing in between (which is very unusual) it is also possible these notes were reported in error and don't actually exist.
I’ve heard to look at the front plate letter/number. If the letter is twice as big as the number, it’s a 32-subject sheet. If the letter and number are the same size, it’s a 50-subject sheet.