What is the Difference between
 

So what are the differences between a "M" caliper on a stock 74 911 and an "A" caliper on a stock 78 sc.

Reason I ask, I am selling some rebuild kits for "A" calipers and the person that is interested has "M" calipers. My guess is they are not interchangeable but I am curious now.

Both appear to be steel units, do the "A's" have a bigger piston?

Thanks

The rebuild kit part numbers are different.

But I think the pistons are the same size, they just use different size pads. The pistons themselves are probably shaped differently, or the caliper shape where the dust seals attach might be different.

I know that the early calipers have an internal retracting mechanism on the backside of the piston and the caliper has a pin sticking out that the piston slides onto. The later calipers don't have this and simply rely on the square seal to do the retracting. My guess is that the seals are different due to this fact.