Here is a lug, the black part. They're not all like this, some are simpler. Basically, they are collars at the joints of the frame tubes that the tubing inserts into. Double butting is harder (nearly impossible, unless you have xray vision) to spot on sight, it helps to read the tubing sticker. But old Reynolds stickers in particular tend to flake off, so it helps to know the shape and location of the stickers, sometimes you can tell by a "ghost" of a sticker.
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