A standard PMR comes with rubber eye covers and rubber detents, they hold the balls in the breach, Kila's are delrin and are aluminium eye plates so are more durable, they make it so you don't have to buy new detents again.
detents are basically there to stop you firing 2 balls whih will cause break in either your breach or you barrel.
the ball sits between the bolt and the detents (1 on each side of the gun) and the ball cannot move forwards past the dentets as it hasnt got enough force. This will stop another ball falling behind it so you dont have 2 balls in you breach (part of the gun in front of the bolt)
detent rubber caps need to be changed every now and then as they get worn down and they work less and less until they simpy dont work