What’s the difference between BA15S and BAU15S Base?

The BA15S and BAU15S are both Bayonet base with a diameter of 15mm (0.56in) with a single contact on the bottom of the base

BA15S (1156) vs BAU15S (7507 PY21W) base

While both bases have two lugs on the side of the base the difference between them is that BA15s have the lugs at 180 degree and BAU15s has the lugs at 150 degree apart.