Old Latitude CPx power adapter pin configuration

Hi,

I'm trying to find out some information about an old laptop adapter. The part number is 9364u and it looks like this:

Notice the three-pronged pin configuration.

I'm trying to find out what this pin configuration is (which one leads to ground, and which are +/- 20V). Does anyone know where to find datasheets for the adapter or do they know how to find this out?

 - R.R.G