I've never seen these before the 70s, not that I have so much experience. My all-original 74 Hetchins Spyder has swaged spokes, nickel-plated steel.
As for the reason, Brandt's book covers it well:
"Spokes are in pure tension at midspan where they do not need to resist bending, so the can be swaged thinner there without sacrificing strength...the most valuable contribution of swaging is that peak stresses are absored in the straight midsection rather than concentrated in the threads and elbow, thereby substantially reducing fatigue stress. Swaged spokes act like strain screws commonly used in high-performance machinery."
In other words, for the same reason that butting/swaging is used anywhere else. It's a basic mechanical principle.
There's more on this later in the book, but that's the main point.
Steve Maas Long Beach, CA
Tom Sanders wrote:
> Yesterday afternoon while detailing out a set of wheels off a '62 bike that
> I am sure is all original I was surprised to note that it has double butted
> spokes. When did they appear on the scene?
> I am sure it has discussed endlessly (although not within the last few
> years) but I am wondering if there is really a benefit in them...I can't
> believe they save much weight and I believe the Swiss ones even use the same
> amount of metal, but hammer the middle of them in a press or something very
> similar to compress the middle of the spoke, or so I have been told.
> Streamlining? doesn't seem to awfully likely to affect
> streamlining...perhaps the ride?
> Tom Sanders
> Lansing, Mi USA