removing pistons

[rubyman]

I am in the process of removing the pistons on my 750cc engine ( I have been a mechanic for 65 plus years and stripped & rebuilt loads of engines car, lorry & plant but never seen a engine like the 750cc ) its my first experience of this engine, at the bottom of the piston liner there is a metal piece that only lets the conrod push through in a certain position what is this metal for, what does it do, its like a extension of the liner.

thanks rubyman