Abstract: To insure that lamps which have a non-rotation symmetrical feature or strure with respect to aligned bases (e.g., a tipped-off exhaust or pumping stub) are placed in a predetermined preferred operating position when inserted into socket terminals (2a, 2b), the lamp bases are formed with stepped surface abnormalities (10a, 10b), for example, in the form of recesses or projections from a terminal engagement surface (8a, 8b) extending perpendicularly to the lamp axis (A--A); socket springs (2a, 2b) have matching surface abnormalities which interengage with the surface abnormality on the base, for example by being formed with ridges, or depressions. The lamp thus can be inserted in such bases only if the respective surface abnormalities on the lamp bases and in the socket terminals can engage.