Don't know, I just know the part numbers are different. Worked on an S with a ABS light sometime ago and found that right axle had been replaced with one that had ABS serrations in wrong place and didn't line up with sensor. Replaced right axle with 91L axle and ABS worked fine.
I ended up using wrong serrated axle on my non ABS 164B. I still wonder what car that axle was really designed for?