A little long, but here is a detailed post I made on the subject at NastyZ28
http://nastyz28.com/threads/1970-z28...r-gear.315632/
Basically what seems to be the case is the new for 70 F60-15 tires diameter needed a way to comp for the diameter and the reducer was the way.
You should probably disassemble the reducer and repack it when you get it. On mine the grease had solidified over time. Even an NOS one is 43 years old now.
Tech page on the reducers:
http://garbee.net/~cabell/stewartwarner.htm
If you want to "date" one of these (that sounds wierd) then here:
http://nastyz28.com/threads/muncie-s...2#post-3551052