But, Wait!--There's more!
I was going to go out & dig around in the closed in back patio/ ham shack/ lab/ storage room & find my spare Weller base unit to trace the wire that goes to the switch, but since it's going to be 39C (102.2F) today, & it isn't insulated, I chickened out!
Anyway, it just so happens that I don't need to do that if you have a multimeter (DMM or analog).
On ohms range, test between the two black wires.
If the "on/off" switch is "on", you will see a resistance of around 63 ohms.(I checked this on my working unit)
This is the transformer primary.
With the switch "off", you will see an open circuit.(Infinity or "OL")
Leave the switch "off".
Now, test between one terminal of the switch & each black wire in turn.
One terminal will show the transformer primary resistance (63 ohms) back to one of the black wires & infinity to the other.
The latter will be the the switched connection, which should be connected to active.
If you pick the other switch terminal first, it will show very close to zero ohms to one black wire, & infinity to the other.
In this case, the one with close to zero is the switched connection.