Damn Ted, I remember 1948 too! That's when I got my first set of aluminum sew-up rims. I don't remember the brand, but they were balsa filled. They had no eyelets so you could look in and see the balsa. The spoke holes were only slightly dimpled for the nipple heads. Whoever built up the wheels (with Harden "bacon slicers") used longer spokes than he should have, so that when I trued the wheels after each as I had to, the spokes eventually protruded beyond the nipples and cut the sew-up threads, allowing the tires to split open. The constant truing was probably mostly caused by my racing on (wet or at least damp) grass tracks. Instead of rebuilding the wheels with the proper length spokes, I just removed the tires and ground the spoke ends flush with the grove in the nipples. I only had to do that three times. Those rims held up that way until the 1949 ABLA Nationals in Balboa Park when another racer came down off the banking, misjudged my speed and put a pedal into my front spokes. By that time I had a spare set of wheels with hollow alloy rims, Fiamme I think, so I could complete the meet.
Mark
Mark Fulton Redwood City California USA
PS: When I say "first set of aluminum..." Before that my sew-up rims were wooden. I had to true them constantly too, probably because of the grass-track dampness warping the wood. I'm only guessing about the dampness affecting the balsa-wood-filled rims. Maybe the spokes in those days stretched more than today?