Single Contact (S.C.) Prefocus Base P15s30B has one contact on the bottom of the base. The base itself has a diameter of 15mm (0.59"). The P15s30B Base has a prefocus ring with a diameter of approximately 30mm (1.18") that encircles the base. Prefocus Base Bulbs with the letter "B" indicates that the distance from
the bottom of the focus ring or collar to the bottom of the base
contact is 9/16. Bulbs that belong to this category include but are not limited to #1321,#1327,#1501,#1561,55A/S8,77A/S8 and BRS and BXA.