"O degree 1 lb sleeping bag" is marketing-speak for a 40 degree 1 lb sleeping bag.

I have a 1 lb 4 oz 25F quilt. Accurately rated, as I have indeed used it multiple times into the low 20F range. In fact, I'd guess it may be warmer, as I tend to get cold easy, so perhaps a warm sleeper would get to 20F with it?

I also have a 0 degree underquilt that weighs 21 oz. But you could not use it without a hammock and similarly rated top quilt, so mentioning it was probably unnecessary.