The black ones are considered a -20ish, the white ones are -50ish. The white ones have an extra layer of insulation in them.
There is a special milsurp wool sock for them that is very long, and is designed to be folded down over the top of the boot so it doesn't sag in them.
The black ones are considerably smaller than the white ones, and lighter as well. A size 8 white one is bigger than a size 10 black one. ETA- I am referring to the outside, overall size of the boot, not the general trend to run large or small according to size that is mentioned later.
Open the valve, put the boots in the cold, then close it. Part of what the boots do is expand when you heat the air between the layers, and it helps make them warmer. If you leave the valve open you defeat that feature. Some people blow air into them through the valve, but that isn't really what it is for.
Not all of them have the valve, it doesn't really matter if they do or not.
You can buy them online pretty easy if you do a search for them.
They are considerably heavier than other boots rated for the same temps, but they are watertight inside and out, so if water comes over the top, you can dump it out and your GTG, as it won't soak into the insulation.
They are really comfortable, but they will scrape all the hair off your leg where the top rubs against your calf, and they are hard to pick up and put down all day. Once you get over those issues, they are like wearing slippers.
The only people that won't laugh at you for wearing them is other folks wearing bunny boots.
They have a ridge at the heel for the cable bindings on the old milsurp skis.
If you put cleats on them, you will need the biggest sizes they make.
Once you get used to them, it is really hard to go back to anything else.