I am sorry, but I was not able to find the original measurements from the seller. Mine is a size 40 and my actual chest measurement is 39" The jacket fits closely but is not restrictive because of the shoulder blade and armpit gussets. So I would say true-to-size if you are reasonably lean. If you are carrying weight, I think you would need to size up.
Also, if you want to regularly wear a heavy sweater underneath. I am o.k. with a shetland, but not anything chunkier.
Are you able to post the measurements of your size 40? I am also at a 39" chest and am a lean build. I am around an 18 1/2" shoulder width.
Can you please post or pm me the pit to pit, shoulder width and back length from base of collar to the bottom?