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NOTE: Before you remove the original distributor, line up your timing marks. For details on how to install Distributor & set engine timing see Ignition Timing.
The timing-adjustment plate on the bottom of the distributor needs to match the engine.
TERMS
Early A-series engine: 1967-1973 (front distributor engines)
* A10 & early A12
Late A-series engine: 1974-up (mid-distributor engines)
* Late A12 & all A13, A14, A15
Left: Late A-engine distributor. Right: Early A-engine version
As you can see in this photo, the Early plate has the adjustment-slot on the opposite side compared with the Late type.
Now, either distributor will fit into either engine, and you can rotate the distributor relative to the engine, but you cannot get the slots to line up correctly in an Early using Late adjustment plate, and vice-versa.
Left: Early A-engine. Right: Late A-engine
It looks like the angle of the plate-to-distributor bolt is at a different angle, but that turns out to be not a problem.