This article is from the VW Technical FAQ, by with numerous contributions by Jan Vandenbrande others.
A: Each part number is composed of nine numbers in three groups, followed
optionally with a letter suffix (taken from WolfSport's catalog):
vvv ggg ppp [s]
vvv: Vehicle type ggg: PRIMARY INDEX
171 = R/G I 100-199 = Engine/Cooling/Clutch
161 = Jetta I 200-299 = Exhaust, Fuel Tank
165 = Jetta II 300-399 = Transmission/Transaxle
261 = 16V GTI 400-499 = Front Axle/Suspension
531 = Scirocco I 500-599 = Rear Axle
535 = Scirocco II 600-699 = Brake
155 = Convertible 700-799 = Cables, Bumpers and Foot control
191 = Golf 800-899 = Body/Interior
900-999 = Electrical, ignition, fuel injection
 
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