At some point during Twin Cam production there was a design change in the Dunlop master cylinder to add a second seal on the piston. Apparently this change applies to both the brake and clutch master cylinders. The first picture (cropped from the Service Parts List) shows the early configuration with a single "O" ring seal (10) near the pushrod end of the pistion (4).

Picture below left shows the early piston. Picture below right shows the later piston. The illustrations give the dimensions. The early piston can be machined to accept the later seal set.

In retrospect it appears that the newer design is (was) intended to superceded the earlier design. This could be viewed as a safety issue as well. This means that dealer service parts would only be available for the later type master cylinder. If/when an early type part might fail, it should be replaced by the later type part rather than being repaired. These days that means you may have to machine the early type piston to accept the later type seals, as the early type o-ring is no longer available.

As a point of reference, it is so far known that car number 801 still had the early style pistons.