Hi jiv, none had a threaded barrel for a muzzle break, when they say threaded barrel they are talking about where the barrel attachés to the receiver verses pinned (your pictures don't show that area) Yours is not an early gun, hence stamped trigger guard, non milled bolt carrier, hence spike bayo. No way to date, can only date factory 26 guns.
Nice numbers matching Gun!! Welcome aboard.
edit: As far as worth I would say 250 to 300 in my area....