Here is the breakdown of what the numbers/letters mean. You guys with ZX's that have a "Y" at the end, don't even ask what it stands for, as nobody seems to know. Remember, this is the "MODEL" number, not the VIN. The number breakdown below refers to the number on the plate behind the battery, NOT the one on the dash, stamped into the firewall or b-pillar.
Here is some info straight from the FSM that should help solve the mystery without opening the diff to read the numbers stamped on the ring gear.
Notice the bottom right corner..."J" represents a GL model.
I'm really kinda shocked that nobody uses the "basic" information thats so readily available in the FSM. If you don't have one, GET ONE!