The mystery of the misaligned turntable is solved, and the suspect finally apprehended. Last night I realigned the turntable lead on an actual radius of the turntable. I found the radius by drawing a line from the centre of the turntable and then measuring half a gauge-width to set the first rail.
It looks like I may not have to move the first roundhouse track, but before I close the issue, I need to make some further adjustments to the turntable height. I thought I had fixed this issue months ago, but it turns out you really want to have the final ties in place before you go declaring victory on the turntable level. With the final ties in place, and especially with some rails pinned down, I can see there is a step.
Unfortunately, this means a shim at the centre of the turntable, and that is the rather enigmatic pair of pieces of brass above. I’m looking forward to reading this post in ten years when I’m trying to debug some issue with connectivity or alignment, and slapping my forehead.
Nothing about the shim is measured – they were cut from a template to match the holes in the foot of the turntable, and then roughed out with my Dremel. If I was keen to move the lazy Suzan, I would solder them in place, but I’m not going to move it, and so, I glued them to the foot of the turntable itself.
Unfortunately, the shim means that the wheels are not going to contact the rails. I will have to devise some sort of support between the girders on the rail to level the turntable. If I were doing the project over again, I would either engineer a way to adjust the ride height of the wheels, or leave the whole sides — including the wheels — off until the ties and rails are in place, riding on a slide.