When you install sapphire, you can usually use the gakset that is already there (just make sure it is clean). Sometimes, a new gasket is needed, then a genuine Seiko glass gaskets is the best to choose.
Divers have a larger gasket, and it is suaully black. The smaller Seiko5 models have a gasket that is attuned to the dial, so a silver dial, or a white dial will have a white/clear gasket. and a blakc or blue dial will have a black gasket.
SNKL41: white gasket 29mm,
SNKL43: black gasket 29mm,
SNKL45: black gasket 29mm,
SNKE49: white gasket 30mm,
SNKE51: black gasket 30mm,
SNKE53: black gasket 30mm,
SKX007: 31,5mm gasket for diver,
SRPC23: 32mm gasket for diver,
SSC015, SNE435: 32mm gasket solar-diver