Incredibly clever oil furnace burner controller (very perplexing)

This is very clearly one of those gadgets from the electromechanical era that was so simple and reliable, that few changes have been made to it over time. My guess is that maybe it was just a large LDR (Light Dependent Resistor) in series with the coil in an early version, but electronics have been added to allow use of a cheaper LDR and maybe add under/over voltage detection.

I've got to admit that this was a very complex puzzle to get my head round. It achieves so much functionality with so few parts that I had to emulate every fault scenario to see what happened, and even then it wasn't initially clear. The magnetically restrained contacts were the most perplexing.

Here's a functional tour from left to right. There's a bimetallic strip with 20W heating element that causes linear movement of an actuator.
The set of contacts on the left are the shutdown/alarm contacts. If the linear actuator hits the end while the heater remains powered, it will actuate the latching contacts, which will shut the unit down and signal a fault until manually reset.

The middle set of contacts had me confused for a while. They only become active on the return stroke when the oil valve contact is closed. It creates a time lag for the second stage oil valve if used.

The oil valve contact is super-complicated. It is normally activated by the thermal linear actuator, but can be locked against actuation my a premature magnetic field on the flame detector coil. If the flame detector coil is active at the end of stroke of the linear actuator, the oil valve contact is held closed by a plastic pin, even when the linear actuator has retracted.
The power to the bimetallic linear actuator's heater is switched through a contact on the flame sensor coil. It can only open when a successful ignition sequence has been completed.

This was another piece of treasure from a dumpster-dive that yielded an entire oil burner assembly.

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