Team Associated TR203A Receiver, 27MHz, 2 channel info needed

I own Team Associated TR203A Receiver, 27MHz AM, 2 channel.

I have a question about battery (power) connector, 1st (upper) three pin slot.

There are three pins in slot, so which one is positive (+) and which one
is negative (-) polarity pin of those three pins in upper slot?

I am not using original three pin female power connector but two pin
female power connector, that's why I ask, to avoid polarity mistake.

I know another two slots are for channels CH1 and CH2, no problems there.

Many thanks for help.
Zeljko