Pulled header and silencer and heard something heavy rolling around in my header - I can tip it and it will fall towards the silencer end and I see a heavy bracket squareish with a round cutout - sound familar? What busted loose? Bike is a '74 Dt360A

Does it look like this? 

This was rattling around inside my pipe.. I didn't realize it was loose until I had removed the pipe but it must have been like that even back when I was riding the bike but if it was I never noticed any effect on engine performance so not sure how important it is. I had to cut a hole in the back side of my pipe in order to gain access.  Once I did that I could see tell tale marks where it had once been and welded it back in place, then repaired the hole. It was not fun patching the hole with my old stick welder and glad I cut the hole on the back side where no one will see it but got it done. I've purchased a small wire feed welder since then.

It sure does Jack! So cut  on the back side close to silencer end? 3x5 rectangular cut or bigger? I assume that when header is open its an easy obvious weld point for my welding tech? Thanks again!

Yes, backside near where the silencer goes.
If I remember correctly there was a small bump on the outside which looked like a result of tack welding the baffle in place originally. For sure I saw something there that gave me a clue as to where I needed to cut the hole. But if you find no such mark on your's then you can compare your actual pipe with the pictures I posted and that should give you a good idea where to cut the hole and what size to make it. 

The larger you cut the hole the easier it will be to weld the piece back in place but of course bigger hole means extra welding when patching it back up.

When you get it out of there look inside the pipe for signs of where it was welded before. It was pretty obivous on mine. Plus the shape of the baffle itself corresponds with the shape of the pipe at that point so that should help you place it in the right spot as well.

Perfect, Thanks!

No real harm in leaving it out.  I did not bother trying to weld the baffle back in, mine came out in pieces anyway.  It would probably help quiet the exhaust down a bit if it's worth the trouble to you.