Slant 6 Distributor Hold Down

This picture shows how the later clamps fit on the later blocks. If you look closely, you will see the small ledge cast into the block for the claw to rest on. The older blocks (I am uncertain when this started) do not have the ledge and will need some provision to apply leverage or pressure to the distributor ring to effectively hold everything snug.
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I am wondering why the Doug Dutra book suggested this without going into detail?