How to tell if Engine is A bore or B bore

So I am trying to get some more stock pistons, have located a place, but have no clue how to tell if I need "A" pistons or "B" pistons. Is there a stamp somewhere on the block?

 

On top of the block it is stamped right behind the alternator

 

Possible it is an AB?

 

<-Front of block

BA
----------- Block Seam-------
BA

This would be
#1 B
#2 B
#3 A
#4 A

 

Yeah, that. ^

edit: this is better

 

Very helpful, thanks guys!

 

You getting the pistons from where I told you to go?

 

Yeah I am. Mine is a ab, ab I believe.

 

How much were they?

 

50+ Shipping.