chamferred and denoted by numeral 47a or 48a in fig3 a partition wall 46 is integrally formed on the circular plate portion 40 at a position between the slits 44 and 45 four equally spaced supporting pins 49 extend in parallel outwardly from the circular plate portion 40 these pins 49 are sized and arranged to mate with the openings 24 of the smaller and larger foot portions 13 and 12 of the afore mentioned bulb holder b each pin 49 has a base portion tightly embedded in the circular plate portion 40 and has a stepped portion 50 at the leading end designated by numeral 52 is a seal ring which is to be disposed about the collar portion 39 of the core structure c as will be understood from fig4 the base structure d is of a monoblock molded plastic member which comprises a hollow major portion 35 and a sleeve portion 28 which are connected at generally right angles with an interposal of a solid portion 25 therebetween the hollows of the major and sleeve portions 35 and 28 are designated by numerals 30 and 29 respectively as is best seen from fig1 the hollow 30 of the major portion 35 has an enlarged entrance 31 which is sized to snugly receive therein the collar portion 39 of the above mentioned core structure c an axially extending key projection 34 is formed on the inner cylindrical.