Is there a front derailer clamp diameter that is near 29.6 or 29.7mm? I have a 1966 Schwinn Super Sport that I bought without a front derailer. The seat tube clamp diameter (i.e., the diameter measured out outside the seat tube, where a derailer would go) is ~29.6mm. Sheldon says the standards are 1 1/8" (28.6 mm) and 1 1/4" (31.8 mm). 

Am I correct in assuming this is some Schwinn-specific diameter that predates those standards? I'm sure I could shim on a 1-1/4 in derailer, but I just want to make sure I'm not missing something obvious that would fit without shimming.
Looks like Saint Sheldon was right, but elsewhere.  http://www.sheldonbrown.com/schwinn-braze.html says under 1962 to 1978: Schwinn Fillet Brazing In the Bicycle Boom


  The fillet-brazed CrMo frame construction of Schwinn's second class of lightweight bicycles - the Superior, Sports Tourer and Super Sport - was unique. The production flow for these bicycles began with frame fabrication in the handbuild part of the Chicago Schwinn factory; a low-volume production area. Frame tubes were mitered in a jig by machine, and then the remaining fabrication work (brazing, filleting, braze-ons, and finishing) was done by hand. The seat and down tubes, as well as the top tube on the largest frame sizes, are slightly oversize at 29.5 mm in diameter. Schwinn called this tubing "1 1/8 inch diameter."


You will be hard-pressed to find a new FD for cheap these days.    Modern components tend to be more 31.8 or 34.9 mm.

Your best option will be to find a used 28.6mm on ebay and carefully file the clamp to fit your tube profile.  A 25.4mm FD would just need a lot more filing to achieve the same fit.

A less-pretty option would be a 31.8mm clamp and then shim it out with a lot of additional material, but that will be uglier, and potential flex under tension will make shifting worse.

You could also use a braze-on adapter and a separate FD mech.