Absolute junk. Each pit below the sealing ring will leak. Pits above the seal comes from water retained in old 'protective' boots. Problem is, every known European brake caliper including aluminum Ferrari and Cobras have OEM chrome or nickel plated mild steel pistons. Check them- they're all strongly magnetic & very hard. Water adsorbed in old brake fluid penetrates the plating and rust-pits like an old bumper.
U.S calipers- OEMS, Wildwood etc) have long used stainless steel pistons (since at least 1967) and that's what we're used to. Unless the brake fluid is decades old, one doesn't get pitting with U.S calipers. Stripping old steel pistons, building up the pits, regrinding and re-plating is cost prohibitive. New pistons are reportedly available for most Girlings but I don't know if they're stainless.