Many thanks for the quick reply and information. I can see it will need 6No 1.5v AA batteries, so the holder in the photo must be a double sided one. The manual site is amazing, and I now have a copy the manual. The manual makes reference to a specially mould battery tray, and I can see there is not a lot of space in the transmitter, so I will see what can be fitted. The mystery will be where the Pos and Neg wires will join the board.
I recall the fully charged transmitter only had a life of about an hour, which was very limiting at the pond side. By the time the Merco 61 was happy, the battery was in the red! With batteries, you can at least swap them out.
As I doubt I would use it in a model, and the units have no real value, it is only curiosity to see what still functions.
Thanks again
David