What is the difference between: BA15S/BA15D/BAU15S/BAU15D/BAY15S/BAY15D?

BA15S Single Filament  

BA15D Dual Filament  

BAU15S Single Filament/Off-Set Pins (Horizontal) 

BAU15D Dual Filament/Off-Set Pins (Horizontal)    

BAY15S Single Filament/Off-Set Pins (Vertical) 

BAY15D Dual Filament/Off-Set Pins (Vertical)