What is this little piece? It's in the end of the boom cylinder. Spins freely, does what? Nothing in the manual about it.
My first instinct is to say that is a "snubber". A snubber is used on some cylinders to slow movement of the piston and rod assembly as it nears the end of its travel. This is to aid in preventing piston and rod damage.
Looking into the cylinder there is a pipe sticking out from the end where the line goes in. Guess when the piston gets there the pipe goes into the end of the rod & does some mystery magic to slow it down. Interesting and not one word in the manual about it. Joe H