Rummaging in the Hollow State Electronics box produced the shapely 0D3 regulator tube with an intact spigot / key post in its base:
Because the glass envelope (1.5 inch = 38.1 mm OD) extends beyond the base (1.125 inch = 28.6 mm OD), the simple base clamp must let the tube extend over the workbench:
There’s no way to clamp that mess in the Sherline, so, rather than freehanding the shell drill, I misused a Dremel slitting wheel to grind away the end of the spigot, which normally extends a bit beyond the pins so you can’t possibly insert the tube into the socket the wrong way:
The missing end exposed the hole in the middle of the post and showed this tube’s evacuation tip didn’t extend into the spigot. Emboldened by that, I continued the mission until the wheel wouldn’t reach any further:
That didn’t work well, but at least I didn’t break anything and nobody will ever see those mauled pin tips.
Obviously, the only way to do this right is to clamp the tube properly and mill the spigot flush with the socket; it’s time for more 3D printing…