Circular Output Amplifier Terminals.

I am guessing speakON speaker connections like on the Benchmark AHB2. But without a picture who knows :)

If you can't give a photo, please give at least the manufacturer and model of the unit that has the "circular" connectors.

I, too, suspect they might be SpeakON connectors, which are excellent, but that's just a guess.

Right…. My very modern 1961 Mac MC240 sports a circular output connection…. 70 Volts / 600 ohm…. 
I would post a picture but the film is out being developed….

While we wait for FotoMat, I posted a picture in my Casa Pacifica virtual system page…. Ignore any dust… 

Cardas Patented Binding Post
https://www.moon-audio.com/products/cardas-cpbp-crs-binding-post

VAC has been using them on their top amps for a while. They’re GREAT for spades, even 2 sets of spades. No wrench required. IIRC they’re Euro compliant because you can’t accidentally cause a short-circuit by touching + and - spades. But you have to drill a hole in them to accept bananas :( 

Edit: Or maybe Speak-on as others mentioned. Those are neat too.