is this a vapor system and what do i have to know...

I am sure more pictures will be wanted. What is the name written on the handle of the rad valve? Are those special 90 connections on the lower rad cond return? Is there a vent pipe sticking into the chimney? What is the printing on the trap/vent/crossover device in the basement? I can't tell you anything other than ask questions.....but someone will recognize this system....I would guess a vapor type of some sort.

Do you recognize the fire extinguisher in your photo #3?
The large ice cream cone looking device with the red ball on top in the white metal holder....it usually has a spring loaded firing pin in the bottom to be activated by the melting of the lead link, just like a sprinkler head. Legend has it that this contains a fluid that will displace the oxygen enough to put out a fire and perhaps suffocate anyone in the room.....maybe carbon tetrachloride, not sure, some people here probably know more.
Anyway, use caution around it or even move it away.

Hopefully, you found your contractor here.

Otherwise, proceed with extreme caution.

dobro23 said:

I am replacing a boiler this week that is a bit unusual compared to others in the area.

Are you the contractor or the homeowner?

Almost surely a vapour system, and I'd be willing to bet that there is some sort of gizmo in what looks like the innocent elbow at the outlet of the radiators.

We will know more if you can get some names, but here are some thoughts -- and questions.

On that fire extinguisher thingy. Haven't seen one in years -- except in the place I care for. The glass bulb is exceedingly fragile, and meant to be. The liquid is carbon tetrachloride -- good call, @JUGHNE -- and it is a hazardous substance, but it will put out a fuel fire right now. Don't breathe it, and try not to breathe the fumes if it gets onto a fire.

On the vapourstat -- I'm pretty sure you will need one; unless I miss my guess, that system will work best at 12 ounces per square inch or less.

You do need vents on the dry returns; that is how the radiators are vented. They can be clustered at the boiler, where the dry returns drop.

The check valve has to go. It was probably a kludge because of excessive boiler pressure.

You may or may not need, or even want, vents on the steam mains. If you go to the ends of the steam mains, is there a trap arranged to connect the end of the steam main with the dry return? If there is, that is sufficient for a vent for the steam main (provided you have those vents on the dry returns!). In some systems, it is necessary that there not be vents on the steam mains. The presence or absence of the crossover traps mentioned above is your clue for that.

You can control each radiator with its valve, but before you start playing with them make sure that your pressure is low and that you have operating crossover traps or main/dry return venting. It is possible that they may be pretty close to right, and it would be a shame to mess them up.