I read up a little on the 500Mils(Military Specs) as i used to have one and it was a 20" 5+1 cylinder bore,brass beaded,aluminum safety button and trigger guard,heavy barrel ect. I would assume any 590's ordered would be pretty plane jane with the basic config as the 500Mils besides the 7+1 full length magazine tube and heavy barrel,and bayonet lug.
Supposedly there were only a few thousand of the ones marked 500MILS and some where issued at US Embassy's.
Supposedly there were only a few thousand of the ones marked 500MILS and some where issued at US Embassy's.