Nicholas
There are no sleves in the Datsun A series engines. The cylinders are bored direct into the cylinder block casting.

Sleves are used in this type of engine as a repair process. The "A" class sleve is thin & is used where the bore is thin or perhaps cracked, but will still provide support. "B" class sleves are thicker & would be used in the case of a bore with a hole through the side, or with serious core shift. These sleves are more self supporting with regard to resistance to bulging during combustion.

Both types can be used to restore a block that is worn beyond the maximum available oversize back to standard size.
Using 76mm pistons in a 1200 block means boring it out to that size.
Chris