A number of blocks are cut concave on the underside so they fit better on both octagon barrels and round barrels. It leaves just the outer edge of the blocks in contact, which makes them sit better.

Wow, quite the setup.  I break those dovetail cutters because I’m impatient, so I clamp a long wide bar (one for the short and one for the long blocks) in my mill vice, surface the edge of the bar, mill to proper width, tip the head to the correct angle, and cut 7-8 sets at a time using an end mill.  Do the same with lower angle. I then separate them, using a ball end mill cut the radius Vall mention in the bottom, and trim them all to final length.  My ends aren’t as nice as yours!  Then the holes and other stuff.   

Still a tedious bunch of operations and I glad when I’m done!

Jim looks like a good solid set up to me. Good fixtures are worth their weight in gold doing small parts like that.

Making chips is always fun.

The Tool and a Base that's off center.  Disclaimer, I'm not making Bases but trying to true them up.  Though the machining of a Base looks interesting.  Made a Cutter for the "Smiley Face" but would buy one if I could.  Simple to use, drop it in the Vice, Sweep the Jaws to find center, Pre Set the Boring Head and what use to be an hours plus job isn't.  David