Eastern Railroad Discussion > PRR....HELP!!!

I recently acquired a PRR brass switch key from the "knobby bow" era. Meaning it has the bumps on the outside of the bow of the key. I have been looking for someone who can help me decipher the single letter markings on the back. I have a "P" and a "Z" and have seen a "T" floating around. from what I have gathered, these letters stood for the division that these keys were handed out?
Please, serious help only. This is driving me nuts! Thanks in advance.

Ken Larson
C&NW/UP Chicago

P.S. See photo for help.

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