The 27s I have seen, the pre-warquarter stock versions (only a wooden butt like the BSA Standard) have the piston seal held by a screw (maybe 2BA? or 5mm?) through the washer into the end of the piston. The washer can take a few forms ... I have mostly seen leather cup washers with a central hard disc countersunk for the fixing screw but have also seen just two leather discs, each c 3mm thick instead of the cup washer, but again having a countersunk washer for the retaining screw.
I do not have experience of the later (newer) mod 27 but believe it too had a leather cup washer held by a screw.
Note: the early 27s have a very basic trigger while the later 27s could well have the feared Diana 3 ball version.
There will be a strip guide to the early 27 in 'Idiots Guide' posts 73, 74, 136. Later 27 is post 123.

Cheers, Phil