That does indeed look like the remnants of a post 1974 Reynolds decal, but you can't tell if it for plain gauge or butted main tubes. I'm assuming it is metric tubing in which case 26.4 to 26.6mm seat posts would be standard fit for the most common gauge of a butted seat tube. However, since you are refinishing the frame, the definitive test is to examine the ends of the tubes after they have been stripped of the paint and primer. Reynolds typically stamped one end of the tube with their name and the tube gauge(s), so frame builders would know to cut from the opposite end, which had the longer butt, Since the seat tube is single butted, the stamp should be found just above the bottom bracket shell. Sometimes the stamping can be quite faint but if it's there, you have definitive proof.