This is a 40-series Integrated IS52 Bottom headset that has a Crown Race diameter of 39.79mm and a bottom stack height of 1mm.
Strong seal, can cross with other 40 series bearings and headset parts
Bearings are black oxide sealed
Durable steel crown race with low-friction face-seals
Compatibility: 1-1/8" straight steerer tubes to tapered 52mm 1.5" IS bottom head-tubes, fork with 39-40 mm crown race seat