They were gas welded. It uses the same sort of oxy / acetylene torch that is used for lugged or fillet brazed construction. Rather than heating the joint just hot enough to melt the brass or silver filler, they get it hot enough to actually melt the steel tubes and steel filler rod. The finished result isn't really much different than any other sort of steel welding (TIG, MIG, Arc, etc.). This is how the majority of steel aircraft frames were welded until Heliarc (TIG) welding became popular in the 60's. The advantage, or possible downside to gas welding is how hot the whole joint gets, and how long it stays hot. Other methods of welding are faster, allowing for the joint to be finished with less heat input, and less heating of the surrounding tubes. This can be both good and bad. The big heat input is nice, in that everything is almost the same temperature, so warping and resisdual stresses are much lower. This would probably also help with fatigue life. It would be bad in that the heat effected zone is usually softer and weaker than the cold worked tubing that hasn't been heated to transition temperatures. Also, some steels can become brittle if kept hot for too long in an oxidizing atmosphere. (I was under the impression that 531 ( a Chrome - Manganese steel) was one of these "hot short" steels. I believe CrMo steel, like 4130, is better in this regard. )