I tried a search on the topic, but could not find an answer my question.  Looking at a Colt illustrated parts catalog, I do not see any reference to a barrel extension or barrel indexing pin, it seems to be all one part (part of the barrel assembly).  When I look at other manufacturers (Bushmaster/RRA) I notice that they have such a part listed.  What is the real deal with these parts?  Are they separate parts or just part of the barrel assembly?  Thanks.

Barrel extensions are seperate parts, screwed onto the threaded breech end of the barrel. The index pin fits in a hole in the extension. For all practical purposes, these parts are of no use to the 'kitchen table' gunsmith. The extension is normally mounted on the barrel first, then the chamber is cut/reamed to correct headspace for that extension. Without a lathe and some building/barreling/chambering knowlege, extensions are of little use. No, they will not interchange like other AR parts.

When you buy a barrel for an AR-15, generally the barrel, extension, index pin front sight base handguard cap, with a barrel nut on there is considered all one part.  You may buy it as a "barrel assembly" which may include the entire delta ring assembly, front sight post, detent and spring, and flash hider, sometimes handguard and gas tube w/ pin.

Barrel extension + WECSOGer (Wyle E. Coyote School of Gunsmithing) = BAD.  

So many variables go into the relationship between the barrel and the extension.  Unless you are very familiar with the construction and function of the AR gas system and chambering, don't mess with them.

Thanks for all the replies.  Its kind of what I figured (it comes pre-attached to a barrel for all of us normal joes, but can also be purchased separately for the serious enthiusiast who wants to do all the fitting).  Thanks again.