Some are made by SA inc., Some are made in Koreafor Red Cloud/Red white and blue, and last but not least China found on Polytechs,Norinco's and some Federal Ordnance rifles.
I have had good luck with the Chinese Op rods and have heard good things about those made in Korea,but have not tried any.
SA inc. op rods are not so great , but they do have the lifetime warranty .