nodster-
Very nice job on the re-paint and a lovely colour combination. You can tell if the tubing is hi-tensile by looking down the fork steerer. You will see a raised seam with hi-tensile and a uniformly smooth cylinder with 531. Check the chainstays as well. Use a torch to look inside the tubing. If hi-tensile, there will be a seam. Rotate your finger to check along the top of the tube where you otherwise might not be able to see. At this point, you will know if the frame and fork are high tensile or 531. Frame weight will help as well. 531 will come in at about 2,500 - 2,800 gms and hi-tensile will usually be about 700 to 800 gms more. Hope this helps.