Early 1990 Does anybody know?
 

What is/was the actual difference in the slave cylinders from the early 90 to the late 90?

Is this a trivia question?
I wasn't even aware of a difference.

No, there is a difference in the part numbers for the early cars.

yeah, here it is:

I would doubt if they could make the flare nut fitting reversed. I have found some that the bleeder is a ball with a concave stem as compared to a non-ball with a convex stem, but I want to make sure of any other fit differences, like the angle at the head of the piston or size differences.

I didn't take them completely apart, but Moss sent me the wrong one - I needed the earlier model, they sent the later one. When I compared the wrong and the right, the only difference I could make out was the flare-nut fitting. All the dimensions seemed idenical. Mine is a '90.

weird. I didn't know there was a difference when I bought mine.