Ummmm, to use a 7812 to regulate negative would be illogical.

Which pin is common ground ? On the 78XX it is in the middle. If it is off to the side that is actually a 7912.

I don't care what it says on it. I'd bet it is a 7912. There is no reason for the engineer to **** around and go through what it would take to use a 7812 there. If it is actually a negative regulator with the ground pin in the middle then it is custom, or a different series. It is actually possible.....but.

They love to use custom parts to create a captive market, but I can think of a whole lot of other places in a PS like that to accomplish that, the standup board with the black conformal coating would do it.

Look at the board, I think you are going to find the ground off to one side, which means you need a 7912 no matter what the numbers say.

Take it from me, I sometimes don't even look at part numbers anymore, I just look at where something is in the circuit and usually know what is needed there. Sometimes I do have to look it up, but not all that often at least when it comes to something like this.