Ron Blanton taught Frame Building in the mid-eighties at California State University, Fresno. He was a professor in Industrial Technology. I had the pleasure of taking his class, which inspired me to this day. I may have watched him build your frame. My guess, the inside of your tubes are coated in primer. He would pour it into the frame and turn it about until the tubes where coated, then pour out the remaining primer. Attention to detail. His craft was Jewelry Making, but taught himself how to make a bicycle after being dissatisfied with what was on the market. He passed away in 2009.