It's clearly one of Gerald's frames, and it obviously was built up. There's a ding on the down tube, and I saved it from the scrap heap. No tubing sticker - I don't recollect if I checked the seat tube dia, I don't know what it weighs, it's just been sitting in my junk box for 30 plus years as I've never gottenaroundtodoingsomething with it.
There's a few here who know more about SBDU and its products than I. Thoughts?
From what I understand they started in 1974 and made ca 1000 frames per year, numbered sequentially; so this would be quite early, like 1975 or so. No fork?
Reynolds 753 was available in the mid 70s, but only in metric diameters. If it's English diameter, it's likely 531 or 531 "Special Lightweight."