Ok, here's the "skinny". I SCREWED UP !

I shoot LH and a buddy of mine came across a LH Rem 1100 12 ga. He told me to take it home and shoot it, see if I liked it, and then make him an offer on it. I really didn't even want the darn thing, but I thought "what the heck". It is LH, right?
I shot it 3 times while at his house. Well..... I walked out in my backyard to shoot some clays a bit ago, and the first shot "blew" the "poly choke" off of it. At least that's what my buddy called it. Waist high grass across the fence, and no idea how far it might/could have went.
I have no idea how old this gun is. The choke can be dialed from improved, modified, full, x-tra full, slug, etc...... It screws on externally.
Does anyone know what this choke is actually called, and where I might find another to replace it. Now all I have is a bare "threaded" barrell end!
Any help/advice is greatly appreciated............. Derek.