My colt AR's have a pivot pin where you simply screw it together from both sides.  I have an Oly upper that I fit on there with one of those adjustable pins, which is flush on one side.

Now I'm building a bushmaster and noticed the pin I bought from them looks completely different.   I'm still putting together the upper and haven't bought the lower receiver yet.  Am I missing something.  How does this thing stay in place?  I guess I need to get a "pivot detent" and spring?  

btt

How does this thing stay in place? I guess I need to get a "pivot detent" and spring?

Yes. There is a closed ended groove in the pin. The detent slides in the groove and locks at either end holding it in.