Monitor/balancers, pre-bms era.
These where pretty common on older setups. Those likely just start burning off current above some arbitrary voltage.
The more sophisticated ones had a single signal cable that could engage a control circuit. Each was connected to the next in series. At the ends of the series, those could be connected to a relay or an input. If any of the monitors observed an errant voltage, they could open or close the circuit to trigger some sort of secondary cut off.