This will be an SP1963 service kit. The fact you have the additional seal suggests this is a TVS-Girling kit produced in India. The far more expensive
EU sourced Lucas Girling kit no longer includes the additional seal, at least it hasn't been present in any of the kits I've seen in the past 5/6
years.
The additional seal was provided on master cylinders supplied until the early 1960s - I'm not sure exactly when it was deleted, but it was certainly
present on the 1959 and 1960 Heralds. The seal fits into a groove at the end of the piston, quite handy for preventing a failed main seal dumping it's
fluid all over the bulkhead, but there's no real operational need for it.
Cheers,
Bill.
http://www.rarebits4clasics.co.uk