BINGO! The penny has dropped Gary 👍 I thought I could see some black wire poking out of the white tube. Put the red plug on that and connect to the socket and all should be well 😀
The black wire coming out of the RX at the opposite end to the servo sockets IS your antenna.
I think the builder has done exactly what I did when fitting a 40MHz antenna to my U25 sub.
To make life easier if he needed to remove the RX for any reason he fitted the plug & socket in the antenna wire. Just as I did when leading my sub antenna out of the watertight hull and up to the removable deck.
How long is the 'white' tube coming through the deck?
At 27MHz the typical 1/8 wavelength RX antenna is 54". The so called electrical length.
Half or so of that is usually in a tightly wound so called loading coil inside the RX case.
So the total physical length of the antenna 'outside the box' should be around 27".
But please not all tangled up inside the boat with other wires and certainly not below the waterline. Keep the antenna wire as far away as possible from the motor and battery wires.
AM receivers are especially susceptible to interference from electric motors!!
Spent 40 years working in radio communications development and system design, mostly for cellular radio and naval ships. So I have some idea about antennas😉
Hope this gets you up and running, and your 'dignal' improves and your cold gets better😁
BTW; your RX is part of a Sanwa Saber RC set, Similar to my 35MHz 6ch Sanwa Conquest.
Cheers, Doug 😎