The entire status of a large volume of products in the form of large sheets of fabric (or other flexible material) each wrapped around a core incorporating an electronic tag can easily and accurately be distinguished via an antenna of an external reading device. Each core includes an inner tube 30 containing an electronic tag 20, and an outer tube 10 within which the inner tube 30 is free to move lengthwise. When the two tubes are viewed in a plane orthogonal to a common longitudinal axis, the cross section of the inner tube is preferably complementary to that of the outer tube such that an outer surface of the inner tube 30 is surrounded by a similarly shaped internal surface of the outer tube 10, and the inner tube is constrained to move inside the outer tube along said common axis. A weight may be provided within the inner tube 30. Alternatively, the inner tube may be filled with a liquid of a slightly high specific gravity so that the electronic tag 20 floats. When a plurality of outer tubes 10 are aligned, their electronic tags 20 can all be aligned in similar positions.