OK, this is NOT going to be the definitive, all-answering, report on the two styles of slant six distributor mounts (early vs late). These are my observations based on numerous early blocks and one late block. The pictures are of 1973 engine (early style mount) and a 1979 engine (late style mount). Just to be clear, when I say early style, I am referring to the slotted plate that bolts to the distributor and holds the distributor in place with a bolt in the slot. The late style is the one most people refer to as the 'claw' and holds the distributer down by clamping down on a round plate bolted to the distributor.
1973 Block
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Next: Part 2 covers the 1979 block.