while tubular rims were always doublewall, I don't think doublewall clincher rims were available until the mid 80s. My old Motobecane hanging in the garage, originally a tubular bike, still has 700c superchampion 'gentlemen' rims from the early 80s, and I'm not sure those are double wall, they were about the best rim you could get back then. Its been so long since I slapped any tires on that, that I can't remember offhand what they look like inside.