The last 20 years or so of Jack Taylors used a unique seatpost clamp bolt. After they quit using the 1/4" Whitworth nut and bolt, they used a socket head screw that screws into a threaded clamp on the frame with a unique british thread called BSF 5/16" x 22. Your local hardware store only has 5/16" x 20 or 24. I needed one so I went looking for them and found some. If you need one for your Jack Taylor or other British bike, I have them in Stainless for $12.00 each or black oxide for $2.00 each. They take a 7/32" allen wrench by the way. The stainless are the exact length of 3/4" but the black ones are 1" long, easily cut.
Bob Freeman
Elliott Bay Bicycles
2116 Western Ave
Seattle, WA 98121
206-441-8144
Home of Davidson Handbuilt Bicycles