Outlets for audio speakers?
My guess also
Possibly an outlet antennae outlet from the TV antennae. Raymond Wand Alton, ON
Used for an Amplifier or a Router control module.
A ham Radio ?
It’s a frequency filter.
Look at the URL extension for a hint.
Certified Inspection Agency?
This outlet may be for AC incoming power cables to be connected directly to the Main Isolator!
This is a 300 ohm tv isolation transformer that was used in high security applications to verify that distribution wiring was not tampered with. It also blocked some information from a TV or other devices from re-radiating out the TV antenna system.
So, Nick what is it?
I will be starting a section of NACHI.org titled “Can you guess what this is?” It will have pictures of stuff you might see on an inspection once in your career.