The rear axle stays to seat tube collet being bent @ center around the seat tube look like collets seen on late 30s early 40s Monark skirts; but, the Monark axle stays curve, going down... yours are straight down; and, the drop-out is different. Monark employed that same collet to attach 4 and 5 bars to the head tube, as well. Your fork has a triple layer crown; Monark's were double layer spaced wider. The Monark used a seat tube shim and a 5/8" stem that anchored with a long bolt and wedge similar to a handlebar stem; Shelby and maybe some others did, too. Your present seat post stem is not the type you asked about.??? Don't think you have a Monark; just sayin' where I've seen that collet.