What is the difference between the slave cylinder on a 84-85 12a compaired to a 13b. can I use either one?

 

I think it depends on whether or not the 12A came with rear disks, i.e. a GSL. That's true for the master cylinder at least.

 

I rebuilt both of my slave cylinders awhile back on my SE and GSL and did notice a difference between the two. I did not take the time to figure in what aspects. Maybe it has something to do with the mounting locations in relation to the clutch forks.