Levelable and floor-decoupled 'blind' cassette for creating an inspection opening of nominal size 4 in dry or wet-cleaned floors indoors, for installation in underfloor boxes UZD250-3 or UZD350-3, each in conjunction with a mounting cover DUG../4 or in UGD250-3/4 or UGD350-3/4. Installation is carried out using leveling angles (order separately) in the aluminum frame profile of the underfloor boxes. Only suitable for smooth floor coverings.