Originally Posted by halfbreedlover
I got the impression that the prefect letters were written in PS/SS, hence why Percy is prefect of Gryffindor. McGonagall hadn't gained access to Ravenclaw Common Room at the time this letter was written, so it may have been correct.
Yes, that may also be the case. It also would show in the book, that not everything planned by the founders,important headmasters, and students had gone as intended. It could also show the insecurities of Hogwarts when Voldemort had began to take control.