[SteveB31]

1000-1999 hubs and drums
2000-2999 brakes
3000-3999 front end
4000-4999 differential
5000-5999 frame
6000-6999 engine
7000-7999 transmission
8000-8999 cooling system
9000-9999 fuel system
1000-10999 charging system
11000-11999 staring system
12000-12999 engine electric
13000-13999 lights
14000-14999 wiring
17000-17999 bumpers, wiper
18000-18999 accessories

The list goes on ...

Prefix before those numbers indicated year and model
Suffix indicated different applications.

For designed this number system with the introduction of the Model A, replacing a bad Model T number system. Same number system still used today.

A rotor is still 12200. Each different rotor has a different prefix and or suffix.