Thanks for those who mailed with suggestions on how to make Rene Herse stems.
Finally, with help from Paul LeMaire, I think I figured it out. For those with a copy of the Rene Herse book by New Cycling, go to page 105 (from the back):
You take a bar of extruded (forged?) aluminum. All tools needed are a drill press, saw, files. You make 7 or so stems at once. You blue the bar so you can put on marks for machining.
You drill the holes for the handlebars. Each time, you also machine away the sides of the stem near that hole, so that the bars will later be surrounded by a raised lip. (This lip, seen in the figure on page 104, I first thought it was an insert. It is not. Whether it creates less of a stress riser for the bars or is simply esthetic, I don't know.)
To machine this, you can either use an end mill that is relieved (cut away) in the center, so it just mills the outside of that lip. (Sort of like a hole saw.) Or you use a planer (?), a spinning tool that holds a lathe cutter at the end. Usually this is used to plane large surfaces (with an end mill), but you could make one that just cuts away the outside of the lip.
Then you turn the bar over and machine the lip on the other side. To locate the tool, you'd use the same drill that was used to drill the handlebar hole.
Then you drill the (angled) hole for the quill tube, enlarge that hole near the top for the bolt head. You also drill the holes at each end of the slot.
Then you set the drill straight and drill the holes that later are part of the handlebar clamp.
Then you cut and file until you have the final shape of the stem, seen in the lower left photo on page 105. Ideally, you have a long-haired apprentice like the one shown in the photo - it'll take a while.
Then you drill the hole for the bell, as well as where the cable hanger is inserted. The cable hanger is riveted and finished off smooth with the side of the stem (see page 104). Thread the hole on top of the expander bolt for the top cap (in 1948, it was only pressed in, in 1952, it was threaded).
Not sure whether the quill is a custom machined part, or simply an alloy seatpin (22 mm diameter - are/were those available?). This is press-fit into the stem. The expander bolt will hold things further together, so no fancy shrink fit needed here.
Polish it all, machine the top cap, engrave it, machine the bolts, chrome them, machine the nuts (alloy), attach the bell, and that is it.
Piece of cake. Anybody interested in making replicas?
Jan Heine, Seattle