I haven’t got a clue as to what this does. The seller said it was supposedly hooked up to a 4208 panel.
Do you have a wiring diagram or anything like that with it?
This might be misinformation, but from the picture and wiring diagram I think it might be a city tie module.
A city tie module monitors the connection between the panel and either the fire department or a monitoring station.
The “remote line tb’l” label would support that theory as well. I think you might be right.