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Using an early A-series distributor in a newer A-series engines is possible by swapping the distributor adjusting plate, along with a slight modification to the bolt slot.
Overview
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.
Early distributor in Late engine (A14 in this photo)
Problem
So, we put the engine-matching plate on the distributor -- then we notice the real problem: the slot on the Late plate doesn't quite match the bolt hole on the Early distributor.
Solution
The solution is easy enough. Use a small round file to widen out a bit of the slot. Or drill a wider spot in the slot. The 2nd slot is used for adjustment, so the exact position of the drilling/widening is not critical.
After this, it's ready to use in your late-model A-series engine! See "how to install Distributor & set timing" at Ignition Timing.