Hi JB,
Reynolds used to identify it's tubing by a stamping such as you described on the short butted end of it's tubes. The 20/23 numbers refers to the gauge of the tubing or more specifically the wall thickness. In millimeters that would be .9mm X .6mm. Somewhere in the 80's both Reynolds and Columbus stopped stamping these numbers into the tubing and had it printed on. It probably had something to do with it being a stress riser. I'm going by memory now since I am not by my inventory of frame tubes but they also put the date on the tubes as well. I clearly remember Columbus doing this when they printed it on but can't remember if the date was stamped in.
Reynolds used to have a lighter top tube matched with a heavier down tube but Columbus would make them all the same. This is why custom builders in the 70's and 80's used to match the slighter lighter Reynolds main tubes with a Columbus rear end. For example, Reynolds would have a 21/24 gauge top tube with a 20/23 down tube. While Columbus SL tubing would be .9 X .6 for both tubes.
Anyway, it is often possible to see these markings if the paint didn't fill it up.
Doug Fattic Niles, Michigan
Question for the experts: Does one normally expect to find a Reynolds stamping on the tubing of an assembled frame? Specifically, while sanding-down my 1960s Tigra (for a redux), I found stamped on both the down tube and top tube the word "REYNOLDS" and on the down tube added "REYNOLDS 531 ??? 20/23." I had not seen that before, and have relied on others to identify whether a bike was indeed built of 531 - if the decals were missing. So again, is it usual or unusal for such a stamping.
(I assume the 20/23 means inside and outside diameter of the tube, but??
Thanks,
JB Froke
Pebble Beach, CA