I've found that's the case with a lot of the Ralph Lauren belt back offerings, too. I guess with lower waisted styles in trousers and lower button stances, they've moved the belt backs down as well?
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What thunderw21 said. The lapel shape, lack of a lapel buttonhole, presence (and size) of the vent, the type of lining, and pocket flaps are the big tells. The label, type of lining, placement of the ****** pocket, etc. are smaller clues.
But the differences are all pretty subtle, really. Have a tailor close the vent, pair it with the right things and it will probably look great. I never pass these '60s belt backs up when I run across them thrifting. They may not be the '30s ones, but most of the time they put interesting, and sometimes very nice twists on the style.

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