the magnetic pickup in the AMC 2.5 and 4.0L YJ or TJ distributor (if that's the one you're asking about) is used for injector opening reference, not for ignition timing.
EDIT: i'm not sure what you mean by the long/short slots, there's a tone ring that passes in front of the cam sensor in the distributor, it's about 1/2 a circle
- a picture could help, i can't visualize what you're asking but next time i'm in the garage i'll have a look at my spare to see if i can figure it out. It might be that we're talking about different models and not the MPFI one.