What are the circular indents on the seat backs for??

As the title says really, are these used for anything? Speaker locations??

Were covering the cards for the back and may cover over them of they're not used for anything.

 

Some busses had fresh air vents in them

 

Yup. For busses with the air duct that ran along the cab door, they're the fresh air exits to passengers in the rear. They were punched through and had vents fitted.

 

just right for a nice speaker I've always thought...... sadly there's only 1 in a westy.

 

I thought so too and have them hiding behind the vents, but it's right against the pillar so 3" only.