I doubt she actually measured the coat. If she had, the p2p would have been about 22.5", give or take .25". Yes, a p2p of 21" would be consistent with a WWII size 40. After the war, the sizes became larger. A p2p on a post war model would be about 21.25", growing to 21.50" or so for size 40 current issue coats.
Many people who don't know much about the p2p measurements, and who don't want to measure, or don't know how, will often just look at the stated chest size, divide by 2, and give that as the p2p measurement. It sounds as if that is what she did. Not helpful at all. In fact it is more harmful than not doing anything.
I'm with you, Peacoat. From what you say, it seems like a Mil-Spec 42R would correspond to a chest size of 43, which is probably perfect for me.
I measured the J Crew Bayswater sizes medium and large and found the p2p measurements to be 23" and 24.5", respectively (hopefully I measured correctly). I like the chest size on the medium - I think what's throwing me off is the sleeve length in relation to the armhole position and the overall length of the coat (it is quite short for me). I think a 42R in the Mil-Spec would be a safe bet, at least for an initial trial.