HAH, just beat myself to the punch...and so I get to flame myself a bit. I did a quick look at the history of aluminum on the web and it seems that Dural is the first of the 2000 series of alloys and developed VERY early in the 20th century. Alloys developed in WW2 and after were some 6000 and mostly the 7000 series. Clearly that doesn't really answer your question as I can't find examples of AL alloy seatposts from the '30s, but theoretically they are possible.