How can you tell the difference between a US Army Moderate and Extreme Cold Weather sleeping bags?

I have a Moderate Cold Weather US Army issue bag from the late 70's/early 80's. It says on the tag that it's for "Moderate" temperatures, but doesn't define what that range is. I rarely use it because it is quite heavy and has a cotton shell. The fill is a mix of polyester and down. It's actually quite warm and I've used it in temps as low as -5F with no problems. It was just such a bear to haul, so I switched to a -20F Eastern Mountain Sports bag I got for a sweet deal. I use that Army bag as a base camp bag when I car camp in winter. I tend not to use it in a tent because the cotton shell could get wet, but in a shelter it's great.