If the Kluson/Sta-Tite tuners on your guitar have been swapped out for a set with larger bushings, that probably means your headstock has been drilled out. Which means that if you want to go back to the vintage tuners, the original bushings aren't going to work. Enter the adapter bushings. They're perfectly sized to fit a 3/8" hole (e.g. Grover Rotomatic), but with the standard 1/4" ID for a Kluson or Sta-Tite style tuner.