I have a piece of literature that I can not attribute but is a mini history of Huffman from 1922-41. It talks about the introduction of the Twin Flex (TF) and says that after the recall of the original design aka "death bike" the TF was re-introduced in April of 1938 as a '39 model. It goes on to say "the model had a straight down tube for a month before it was changed to the double curved tube." This supports what Marty
@cyclingday said about this being an early vs. late thing. The TF is an interesting model consisting of a lot of variety that is often overlooked. Such models as the tank camel back, economy TF--no tank, and a host of other things. This is probably a subject worthy of more study and a monograph at some point but I got other irons in the fire right now. V/r Shawn