If you have this Case back number 6R15-00CO then here you go 31mm for Boley site
Brand Seiko Reference 6R15-00C0
Group Part No Description item no.
crystal 310P55SN02 GLASS(SAPPHIRE)
Otherwise...
Case-part crystal crown bottom gasket glace gasket strap
6R15-00A0 310P55SN02 9K65A5SNS0 FE318BAD2 86565100 D385AG-LK00
6R15-00B0 310P55SN02 9K65A7SNS0 FE318BAD2 86565100 DFS0AW
6R15-00C0 310P55SN02 9K65A5SNS0 FE318BAD2 86565100 L00M011J0
6R15-00D0 320P35SN02 9K65A7SNS0 FE328BAD2 86566881 D380AG
6R15-00E0 320PX5SN02 1Z60B1SNS0 DB0055B01 86622470 DG26AB
6R15-00F0 340P25SN02 9K65A3SNS0 0F290BAD2 86578941 4K31AB
6R15-00G0 310P57HN03 1E70C1SNS0 0J316BA42 86592930 DEN4AZ
6R15-00H0 320P02SN02 9K65A7SNS0 FE328BAD2 86566881 D3G6CG-LK00
6R15-00J0 350P06HB0A 9K55B5SNS0 DH0100B01 86579881 DG67AB
6R15-00K0 350P25HN03 9K55B5SNS0 DH0100B01 86579881 DG71AB
6R15-00L0 350P25HN03 9K55B5SNB1 DH0100B01 86579880 DG70EB
6R15-00M0 350P25HN03 9K55B5SNS0 DH0100B01 86579881 DG71AB
6R15-00N0 350P25HN03 9K55B5SNB1 DH0100B01 86579880 DG72EB
6R15-00P0 350P06HB0B 9K55B5SNB1 DH0100B01 86579880 DG68EB
6R15-00R0 350P25HN03 9K55B5SNB1 DH0100B01 86579880 DG68EB
6R15-00S0 320P02SN02 9K65A7SNG0 FE328BAD2 86566880 D3G6BG
6R15-00T0 310P55SN02 9K65A7SNS0 FE318BAD2 86565100 WDB7AW
6R15-00V0 320A17HN01 9K55B7SNS0 FE318BAD2 86565960 D00G2BL
6R15-00W0 310P55SN02 9K65A5SNG0 FE318BAD2 86179152 D385BG-LK00
6R15-00X0 150P13HN03 9K55B5SNS0 DH0100B01 86582720 DA0H1AM
6R15-00Y0 150P13HN03 9K55B5SNS0 DH0100B01 86582721 DG67AB
6R15-00Z0 350P06HB0C 9K55B5SNB1 DH0100B01 86579880 D0052DL
6R15-01A0 350P25HN03 9K55B5SNS0 DH0100B01 86579880 D0051AL
6R15-01B0 350P25HN03 9K55B5SJB0 DH0100B01 86579880 DG68EB
6R15-01C0 320A19HN01 9K55C5SNS0 FE318BAD2 86565960 D00E1AM-LK00
6R15-01D0 310P57HN03 1E70C5TNA0 0J316BA42 86592930 D00D1DS
6R15-01E0 350AA0HN02 9M50H7SNG0 FE373BA11 XX000
6R15-01F0 300P79HN02 1E70C7SNB0 0G330BA11 86593875 D00M1AM-LK00
6R15-01G0 300P79HN02 1E70C7SNB0 0G330BA11 86593875 D00M1BM-LK00
6R15-01H0 300P79HN02 1E70C7SNB0 0G330BA11 86593875 D00M1DM-LK00
6R15-01J0 FE318BAD2 WDF22AC
6R15-01K0 315P25HN02 1E70D1SNS0 0C3060B0A 86600630 D00X1AM-LK00
6R15-01L0 315P25HN02 1E70D1SNB0 0C3060B0A 86600630 D00Y1DS
6R15-01M0 350P25HN02 9K55B5SNS0 DH0100B01 86582720 D0101AL
6R15-01N0 350P25HN02 9K55B5SNB1 DH0100B01 86582721 D0102DL
6R15-01P0 350P25HN03 9K55B5SNB1 DH0100B01 86582721 D0103DL
6R15-01R0 350P25HN03 9K55B5SNB1 DH0100B01 86582720 D0104DL
6R15-01S0 360A07HN01 9K55D3SNS0 FE358BAD2 D0151AL
6R15-01T0 360A07HN01 9K55D3SNG1 FE358BAD2 D0152BL
6R15-01V0 360A07HN01 9K55D3SNG0 FE358BAD2 D0153FL
6R15-01W0 300PX6S20A 1E80A5SNS0 FE318BAD2 86178610 D0171AL
6R15-01X0 300PX6S20A 1E80A5SNB0 FE318BAD2 86178610 D0161DMLK
6R15-01Y0 300PX6S20A 1E80A5SNZ0 FE318BAD2 86178610 D0211DL
6R15-01Z0 310P55SN02 9K65A5SNZ0 FE318BAD2 86179152 L00M012P0
6R15-02A0 310P55SN02 9K65A5SNB0 FE318BAD2 86565100 M0K41MLK
6R15-02B0 310X09SN03 1E70C1SNS0 0J316BA42 86592930 S004011J0
6R15-02C0 300PX6SN02 1E70C7SNB0 0G330BA11 86593875 D00M1DM-LK00
6R15-02E0 310X09SN03 1E70C1SNB0 0J316BA42 86592930 S005011M0
6R15A-MOVT