These Things ($35) by House of Looptail are in-line-cable-adjusters, helpful little problem-solvers for a myriad of old-school builds, and in particular, quite necessary if you are building a drop bar conversion with cantilever brakes. When installed behind vintage thumbies or bar-end shifters, they will remove cable slop and greatly improve shifting feel, as most olde-tyme derailleurs never came with adjusting barrels, i.e., early Huret, Campagnolo, most Suntour, some Shimano, etc.
WTB and others produced these in the 1980s and early 1990s, but in case you didn’t notice, that was decades ago, so availability is not great. We do know about a few modern solutions, all of which look really dumb on a Prewar Klunk, an ’85 Commando, or a ’91 Jellybean. So House of Looptail made These Things™ to be functional yet understated and to look like they might actually belong on your vintage bicycle.
Check out more at House of Looptail.