Hello, nice to see you back for your second post, look forward to the next in 2028.
It's RGD (not RDG) by the way, it stands for RadioGramophone Developments. It might help when searching
Some info here which suggests a cord was used:
https://www.radios-tv.co.uk/communit...dio-model-516/
The chap who wrote that thread is David Boynes who is "Fernseh" on this forum. Maybe he'll see your request.
Nick.