Just add this to the "life's lessons learned the hard way" file. I recently purchased a beautiful, old school CHP black leather jacket from a vintage online clothing store. Most likely it was custom-made for a motorcycle officer in the 1960's, but it may be from the 1950's - nice Talon zippers, may be horsehide but I'm not sure, etc. It was my size - 50, which was written in pen on the lining of the inside chest pocket. I'm told that was a common practice back in those days. The jacket arrived, and no doubt about it, it's a fine jacket, but way too small for me. It fits more like a 46 I think. I can't send it back because the store will only issue me a credit and they don't have anything else I want. But I like the style of this jacket so much that I just ordered a modified Columbia from Langlitz that should match the vintage one, and actually fit. I'll hold onto the old one, compare my new Langlitz to it, then sell the old one I suppose. But the real lesson for me is: NEVER buy without trying it on. Most of you vintage jacket buffs probably already know that, and I shoulda known, but I let desire overcome common sense.
Actually, the lesson is get the important measurements, ignore the size. I've bought many jackets without trying on. Never been a problem. Good vintage places will give you arm and back length and pit to pit.