Kwstaton - if the previous owner had to mark the plug for correct orientation, then I would consider the plug damaged and should be replaced. There is no way to be certain they marked it correctly without checking all the wiring connections. I would not rely on an unknown mark for something important as trailer wiring and braking.
Time IMO to do some homework. There can be some variation in wiring, but 7 pins in the US are pretty consistent. Go to
http://www.etrailer.com/faq-wiring.aspx and review the information there. Buy a new plug and wire it so the connecting plugs match your vehicle. You can use a voltage tester to see what each female plug position does on your vehicle. Have someone activate the brakes, turn signals and brake controller on your vehicle, and you will see voltage. Make a diagram and wire the new plug to match. You will see a slot on the vehicle female plug that is supposed to allow the male plug to only fit one way. That's why it is there, to prevent incorrect wiring connections.