This article is from the Watercooled VWs General FAQ, by Jan Vandenbrande with numerous contributions by others.
A: The A-1 and A-2 designate the platform/generation the car is based upon.
A-1 (or Mark-1) were the first generation water cooled VWs such as the
Rabbit/Golf I, Jetta I, Cabriolet, and Scirocco I & II.
A-2 based VWs are the Golf II, Jetta II, and the Corrado.
A-3 is the latest platform, which includes the Golf III, Jetta III/Vento,
and the yet to be released new Cabriolet.
VWs based on the same platform share many of the same components such as
suspension components (rear torsion beam, sway bars).
This however does not imply that the engines are the same within one
platform.
Within one platform, many different engines are available (mostly Europe.
The old Golfs came with anything from a 1.1l to an 1.8l, including
diesels), and across different platforms you may also find many of same
engines (the 2.0l 16V is used in the A2 Golf GTIs and the B3 Passats).
Therefore similar engines will share similar parts, and certain
improvements from one platform can often be transformed to the other
(e.g. exhaust manifolds from an 81 GTI to an 84 S).
There's a complete list by Tom H at the end. It includes Euro & US models!
 
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