P=Piso ? Q= Mark ?Mark in the new testament was known as Q ! Mark was written in a prototype form before it was later crafted into the form that we are familiar with. The earlier version was called 'Ur Marcus' and is also known/called 'Q' (for 'Quelle', which is German for the 'source').

Sauce http://www.angelfire.com/wi/famtree/wrotent.html