Socket on the back of some transceivers

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I'm somewhat shopping for a transceiver and some of them have this

Just wondered what it is

Thanks in advance I'm gonna take my test 27 June and looking for something decent that might partially function and I can work on.
At first I kinda ruled out any tubes but now I see that maybe some tubes are more available than output transistors.
So I'm lost but it's all good I don't really want to talk just listen that would be cool.

I'm still overwhelmed I really need a heads-up on what type of radio would be the easiest to find people talking about anything honestly I don't care if it's somebody reading a phone book.
Unfortunately I still want a boat anchor I can play with.


Thanks in advance this place ROCKS you guys THE BEST

 

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Alright well this seems to be a connection for a power cord