I think the missing part of tool #8 is a dial indicator.
Tool #8 would screw into the spark plug hole in the head. Then a dial indicator would slide onto the large hole in part #8.
The bottom of the dial indicator has a wide tube running out the bottom, and a smaller shaft inside of it. It's that wider tube that inserts into tool #8, and is held in place with a set screw.
The shaft above would be screwed onto that smaller shaft of the dial indicator making it long enough to reach down to the top of the piston.
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