The warmest ones (black or whie) have the valve on the side. These were designed for paratroopers (from what I understand). The ones without the valve were for ground troop and have one less layer of wool felt.

I have both, and can really tell the difference. I have actually had my feet get cold wearing the ones without the valve.

Think my dad wore em in Korea. I have no experience with em.

The valves in the side of the boots (black or white) were used to adjust the pressure inside of the boots when at altitude.  These boots are made with a sealed rubber outer and inner that has insulation sandwiched in between.  If you couldnt equalize the pressure they would keep getting tighter with higher altitudes.  Ive had both and as far as I could tell in Northern Michigan the valve made no difference in warmth.  I have seen people ruin MM boots by getting water in through that valve and soaking the insulation, it only has one way in and one way out and it basically will never dry once soaked.  My advice if you do get the ones with valves is to smear a dab of silicone into it so it cant let anything in unless you plan to use them in the mountians or on air planes.

As far as my experience with them Ive been using them now for 20 years (same pair mind you) and I hunt/fish a lot with them.  They are warm and waterproof and durable even if a bit cumbersome.  Ive fallen through some thin ice more than 2 miles from home in December and when I got home I poured steaming water from each boot on the porch before going in.  If anything I tend to over heat in them when moving about.