Help Identify Stainless Steel Automag barrel

I thought this would be the forum to post this on instead of many others. I have what seems to be a pretty old barrel that has one wire detent slot. Beside the heavy weight of the barrel, the inside seems polished/honed and very smooth. Looks like a quality made barrel but I've been trying to identify it to learn of its history. Any help would be greatly appreciated.......thanks in advance.

http://i1103.photobucket.com/albums/...ps2c3f722b.jpg

Interesting barrel, can't say I have seen anyone post this type of barrel before. Is the middle grey area painted or just unpolished? How long?

Reminds me of the old 9" thunderpig, but that had straight oblong ports at the tip.

It's 10" tip to tip