Per my fish scale my 58 cm C to C Colnago SL from about 1983 claims to be 23.5 pounds. This is corrected for the reading I got weighing twenty pounds of barbell weights for calibration. Full Super Record except Look Carbo Pro pedals. The saddle is a steel rail Regal. Clincher wheels with 700 x 23 Continental Grand Prix folding tires. It also has one bottle cage mounted.
I cannot see much possible weight reduction being possible on this bike without new tubular wheels and tires plus a lighter saddle. Not sure what that would save but I would think less than 2 pounds.
Historically a lot of claimed weights have been for a small frame version of a bike with the lightest possible wheels and tires fitted. Many also w/o pedals as they are delivered without them. Other than track bikes or special builds I would expect it would be difficult to get a 1970s or 80s lugged steel frame bike with a reasonable size frame much below 22 pounds.