Each mainboard flex here is made for one Watch Series generation and one case size, from Series 4 through Series 9 and the SE. Sizes changed along the way — 40 and 44 mm up to Series 6 and on the SE, 41 and 45 mm from Series 7 onward — so read the size engraved on the back of the case before choosing. A cable from the neighbouring generation will usually not line up with the connectors.
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Each mainboard flex here is made for one Watch Series generation and one case size, from Series 4 through Series 9 and the SE. Sizes changed along the way — 40 and 44 mm up to Series 6 and on the SE, 41 and 45 mm from Series 7 onward — so read the size engraved on the back of the case before choosing. A cable from the neighbouring generation will usually not line up with the connectors.