ERA makes the rear sight you are looking for. Sold in the
US at New England Custom Guns. It has two hidden screws
for mounting to the barrel and a third to lock the sight.
It can also be soldered on. One standing and two
folders. A well made piece. I am putting one on my rifle.
It matches up with a NECG front sight that takes the
folding white nite sight. This takes just a bit of machine
work to make it right but it is pretty nice. I think NECG
exports out of the US. Both pieces total about 160 USD.
Good Luck
Raff