Ronald - last question first: No earthly idea what the 'double' means. Not 'double action' .. because they aren't. I don't think it is 'double stack' mags, because until recently, virtually all were. So .. you've uncovered a mystery. As to theh serial number debate - Glock ain't talking, and anything you read is 99.9% surely based on nothing more than anecdotal 'evidence'. I've read for years the "US" suffix was on pistols that were made in Austria for export to the US. I've seen Glocks with serial numbers that were apparently 'customized' for a particular police department, for example who bought them .. i.e. "IFIXXXXUS" for the Idaho Falls, Idaho Police dept. There are as many theories floating around as there are urban legends. And .. Glock ain't talking. Bottom line though seems to be it doesn't matter where they're made, they're made of the same materials and to the same standards - which are actually pretty darned good standards. All materials come from Glock/Austria - even for parts made in the US. There is no local procurement of any materials here in the US. The machinery used to make them all comes from Austria. They are truly "equals". Good luck in your quest.

Ronald, information on the internet has been spotty on model numbers between US and Austria made Glocks. I would get accurate information directly from Glock Customer Service at Phone: 770-432-1202 or by email inquiry here : https://us.glock.com/en/contact-us ------ Some info with pictures here : http://smallarmssolutions.com/home/glock-austria-vs-glock-usa-made-pistols