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 breast 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.