mine was the same as the front bolt orings - 1.5x11mm BN70
i use the lurker so i have no back bumper oring. but i put the 1.5x11mm BN70 on and installed the stock bolt, and it seemed to fit and work perfectly.
im also not positive of oring boss's new xsv and infamous bolts use as the back bolt guide bumper to shot the fragile paint. the optional oring. if anyone knows, let me know and ill put it on the list for boss v2 axe.