What is the difference between a BA15D base, BAY15D (1157) base and BA15S(1156)?
Bayonet based abbreviation is BA fallow by a number, which represent the base diameter in mm.
BA15 base is a Bayonet base with a diameter of 15mm (0.59in)
The lower case after the base diameter represent the number of contact on the bottom of the base, and based on the number of contacts there are 2 base types:
d - double contact
s - Single contact