the end face 2 and a seal 6 the cap 5 is concentric with the longitudinal axis 7 of the housing 3 and comprises an annular shell 8 coupling pieces 9 10 11 and 12 which together constitutes a coupling device 13 consist respectively of extensions 14 and 15 of the shell 8 a flange form projection 17 of the seal 6 and a retaining ring 16 the seal is in the form of a sealing plug having a circular sealing face 18 and sealing faces 19 and 20 extending transversely of the longitudinal axis 7 the sealing face 18 cooperates with an internal contact face 21 of the housing 3 the sealing device 6 has a central depression 22 in its side facing the retaining ring 16 the depression 22 which has a diameter 24 fig2 is of approximately the same cross sectional area as an opening 23 in the ring 16 the projection 17 which constitutes the coupling piece 11 and is in the form of a flange extends over the sealing face 18 of the seal 6 and is secured between the extensions 14 and 15 which lie in respective planes which are spaced from each other in the direction of the longitudinal axis 7 and also transversely thereof the extensions 14 and 15 are as shown annular circular projections for securing the seal 6 in the cap 5 the retaining ring 16 is interposed between the seal 6 and the extension 14 the ring 16 has a greater external diameter 25 than an internal diameter 26 fig2 of the extension 14 likewise the diameter 24 of the opening 23 of the ring 16 is smaller than.