That is the putlet of the unloader what is mossing so will also require the non return valve (looks like a bullet), I would not bother trying to source them as those unloaders are crap in my eyes, Ive had loads on machines that have come into me and 99 percent of them have a cracked piston to go with it
As chris says, buy an unloader, preferably a k5.1, you will also need a few other bits like a new inlet fitting which will be a 1/2inch bsp to 3/4 hose tail, also you will need....
3/8 bsp male / male
3/8 bsp male / female
1/2 bsp male to 1/2 hosetail
3 meters of 3/4 inch suction hose
3 meters of 1/2 return hose
and a weighted suction filter
Also worth looking at the pistons for damage