Is that a tapered octagon to round barrel on there? Fine hunk of walnut too. .... I never get tired of this stuff. Thanks JR.....
The barrel was built in Austria many years ago and recently rebored by Delta Gunshop. It's one of the few integral rib half round/octagon that didn't have any rear sight dovetail cut into the barrel. Musta come with a peep.
I wouldn't push your Erfurt too hard. Pretty sure they are small ring, large thread.
There are some more pics of the Mex posted above over here at.
AR Mexican thread 
