The old original main spring (Item No. 10, Illustration No. 9 in the parts book) inside the master cylinder was 88 mm long and had an outside diameter of 20 mm. The replacement main spring (Item 33, no illustration) for the replacement service kit 8G8496 with the new piston with two seals is 100 mm in length, with the same outside diameter and wire thickness. It is thus definitely stronger when compressed. The wire diameter is 1,62 mm for both springs. The later, longer, stronger spring should be used with the later piston with two seals.
In retrospect it appears that the newer design is (was) intended to superseded the earlier design.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.
This is a Dunlop master cylinder for Jaguar, much different housing but same internal parts.
Here is a section drawing of the real Twin cam brake master cylinder.
Click for larger pages (400-KB pdf file).
Pages from Service Parts List, Version 3, showing the change of master cylinder. The earlier assembly BHA4071 has the single O-ring piston and calls for Service Kit SC8196. If SC8196 not available, use SC8496. The later kit includes the new piston and newer style seals and will convert the early master cylinder to the later type BHA4157. It is not necessary to list part numbers for the parts in the kit, as the piece parts were not sold separately. Since the housing is not changed you cannot tell what parts are inside the unit until it is disassembled.
At some point in time (soon after the Twin Cam was built) Girling took over support of Dunlop brakes. Thereafter replacement parts would have the Girling name on the packages.