DC plug pins. Which is positive?
The pin with the tab on the bottom left is the center pin.  The tab at the very top is the sleeve.  The middle pin is a break contact on the sleeve connection.

Most old guitar pedals use center (-) - but not all.  Check for continuity between the center pin of the connector and the battery connector.

You can either ignore the break connection or, if you still want to have the option of a battery inside the pedal, connect whatever lead corresponds to the sleeve to the break connection.  This will disconnect the battery when external power is plugged into the pedal.