It may have been a pin to hold the tubes together. I bought a wrecked PX 10 years ago to rob parts. I cut up several tubes to used as tools. The down tube makes a very nice crown race driver. I cut thru a few joints to check the braze. Well, the top tube had a complete brass rod stuck to one of the head lugs on the inside like they shoved it thru the vent hole and then heated up the joint to braze from the inside??? I could see then why some folks bikes have a rattle they could never find! Dickey Greer West Monroe,La USA Dickey's Recreation Room