The improvements provide for the lugs of the tooth to have stepped guides at the top and bottom which combine with widened end regions for greater reinforcement and with a projecting internal abutment in at least one of said lugs, capable of being guided in a corresponding straight guide of the tooth holder, being arranged, after it is mounted, so as to retain the pin from behind, which pin is arranged in a pin seating arranged generally vertically in the body of the tooth holder, with a gently curved and inclined structure.