the modern rebuild kit for the older cylinders includes a replacement plunger to convert the old 1 ring, 1 cup type rubber seals to the modern 2 ring type.
it will fit and work fine.
please note, these are my own, individual sales, nothing whatsoever to do with my employer, minispares
Also, the brake master cylinder should have the three-legged white nylon, non-return valve. This goes in the cylinder first, so sits at the bottom of the cylinder (top -when it's fitted in the car)