Can I add one more description to this pi**ing contest?
It could represent a co-axial connector.
I use the word co-axial to represent an RF type cable rather than just a screened cable, which may be fine at audio frequencies but have excessive loss at RF.
As the circuit shown by the OP appears to be an amplifier for a low (RF) frequency loop antenna, I will stick with my description of co-ax connector as the output is intended to go to the input of a receiver which I guess will have a co-ax socket.