Abstract: The seal (3) is provided with a plurality of laterally projecting attachment portions (14, 20, 814, 30, 31), located at longitudinally spaced positions along the seal. The rim (R) of the plate is formed with a plurality of recesses (19, 21, 33, 34) which extend laterally from the groove (10) in a direction essentially perpendicular to the groove at the locations of the attachment portions. The recesses, formed as cuts from the edge (15) (15') of the rim (R, R') are open to the terminal edge of the rim (R), so that, for placement or replacement of the seal, the seal, or seal portions, e.g., after loosening from the groove (10), can be readily slid outwardly of the groove by gripping the attachment portions. The seal (3) and the attachment portions are protected by the portion of the rim (R, R') extending beyond the recesses.