What would an “Expansion Tank Relay” been used for historically?

looks like a pressure switch that determines if the system is sufficiently filled from the weight of the water in the pipe to the expansion tank. If it opened a feed valve or shut down the burner is another question.

I can't tell where the pivot is. it kind of looks like the bellows are cracked on the top left. Is there a label inside the cover?

I think — and I could well be wrong! — that the switch is mounted on what looks like it might be a wooden arm which is hooked under something on the left side. The bellows pushes up on that arm, and the spring pulls down. But… without actually standing there fiddling with it, I could be all wrong!

Depending on your level of curiosity. I'd clean off the dust (Red Arrow) on the Mercury switch to see which end the glass tube the contacts poke into the Mercury, probably the wire end but you never know.

Also the wire insulation changed between the Mercury switch and the connections inside the Orange box and the wires that enter the box. So is that a terminal strip, a fuse holder, a relay, other device?

Be careful with non contact electrical testers, if that Mercury switch actually switches the Neutral side of the circuit and the switch is closed (circuit live and active) the non contact electrical tester may not alert until the switch opens. I see taped wires AND also other wires. If you explore, Be safe, open the knife switch and remove the fuses !!!

OK as long as you are sure " It was obviously disconnected for a reason and I’m ok with that. "

I see 3 taped wires and maybe 5 wires going into the conduit. To me it may still be in service. When I see something like that I assume it is live until proven otherwise. Taped wires in this situation is no proof.

" I am leaving well enough alone lol. "

If you don't touch it you will be good.

Looks like it goes into a conduit that heads toward the knife switch disconnect / fuse box.

the 2 pole safety switch for a control transformer doesn't make sense either.

that is an enclosed safety switch, square d invented them, fused in this case. the d stands for detroit. they moved their remaining operations to chicago when we built an expressway through their campus.

they were commonly used as service equipment when 120/240v 30 a services were common.

I believe you have a Honeywell heat generator. Don't make https://www.heatinghelp.com/news-and-media/dead-men-tales/a-50000-dollar-mistake/ because this contraption is filled with mercury, and a lot of it! Click on the podcast link to learn about it.

The original expansion tank on that system was in the attic and may still be there. The Expansion tank switch was to make sure the pressure stayed at a safe level so the water temperature could get above 215° without steaming in order to use less radiators in order to compete with Steam boiler installers.

the baso switch is abandoned, it has a modern combination valve.

looking at the other post and seeing that all the controls switch line voltage it likely had some sort of oil burner until the 50's or so when natural gas became available in the quantity needed for heating.

this looks a lot like a patch from a stack relay in the other post

the baso switch is normally in the low energy wiring to the gas valve.

" the baso switch is normally in the low energy wiring to the gas valve. " That may be true but in this case it does not look like it was wired that way. And at least the newer ones are rated for the higher voltage.

It appears to me the small wires in the upper part of the safety switch is a piece of SJ or appliance cord (Orange Arrow) that goes through a piece of flexible metallic conduit to the control transformer(s). I would say the thermostat and the Gas Valve are 24 VAC. The BASO switch (probably bypassed), Aquastat and the Expansion Tank Relay are all on the 120 VAC side of the circuit and part of the circuit.

What I am not really seeing for example, is if the Aquastat opens the 120 VAC circuit how does that shuts off the primary of the control transformer. Although it appears there is another path to the control transformer(s) through another conduit.

Yellow arrows, another taped wire connection, not dead ended.

Also the Neutral looks Switched and Fused, at least some of the Neutral. It looks like the Neutral to the piece of SJ or appliance cord is under the screw on the line side of the switch.

Thanks for the pictures.

Well I hate to say, the Non-Contact testing in the video proves nothing. If the Hot wire happens to go directly to the primary of the control transformer (the load) all the other wires connecting the Limit and safety switches would be at Neutral potential which usually does not alert a Non-Contact electrical tester.

Also (so far) I see no evidence that the wires to the BASO switch (probably bypassed at that switch), the Aquastat and the Expansion Tank Relay are disconnected and actually out of service.

If you are comfortable doing so, completely remove the Right hand (Neutral) fuse and repeat the test in the video. See if your dead wires are now Hot.

What is the upper transformer for, the Doorbell ? I suspect the thinner wire in the upper half of the box that the colors don't match the incoming power wires only powers the upper transformer.

Does the safety knife switch actually shut off the boiler ? If so the boiler's control transformer is not powered by the thinner wires in the upper half of the safety switch box, since the thinner wires are on the line side of the switch and are not interrupted by the switch or the fuses.