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Finally got enough projects off the bench to get to the toolpost.
The central shaft has an eccentric section that bears on pistons set into the body.
The pistons are spring loaded in, and disassembly is done by pulling out both of the pistons at the same time while holding it upside down. Then the central shaft falls out. It's easier to pull them out if you take the pads off the top of the pistons, put the screws back in and pull on the screws.
There are no adjustments. The difference in timing appears to be due to both pistons being different length.
I swapped the pistons and now the handle stops at the same place, which is with the handle facing the tailstock. A big improvement.
The surface of the central shaft is not finished very well, so I might chuck it up and use emery cloth to smooth it out, which would give a better feel to the locking mechanism.
If you want the handle to turn less, you can add a thin washer under the cap on the piston, but making it clock more requires removing some metal from the pistons.