Generation 2 of B-31 and B-31S upper handguards is now available. They are designed for B-30 and B-30U upper handguards. New modification can be also used together with "Sport-1" kit, in particular with B-10L handguard and DL-1 additional attachment (instead ob B-31L upper handguard). 
Note: older modifications of B-31 and B-31S are not compatible with "Sport" kit!
B-31 upper handguard is designed for rifles with longer barrels (415 mm). B-31S upper handguard is for rifles with shorter barrels.