Cool. I think I'll try opening up the bore a bit, and also seeing if the two holes are aligned properly. Here's an exploded diagram of the head. This is from a G0704, but the setup is the same, the part sizes are just a bit bigger on my mill:
Attachment 278114
I've been talking about 205, 206, and 207. Really, they just stabilize the top part of the spline of the spindle (246) which has the belt drive pulley bolted to the top. I was thinking it didn't need a whole lot of pre-load either, as most of the cutting force is supported by 296 and 248. Those were replaced with angular contact bearings and are doing great.