In article ,
says...

https://dl.dropboxusercontent.com/u/...070%20Caps.jpg

In a Kenwood receiver. It is steel, not aluminum.

What is its purpose ?

Not sure, I don't think it's being used a Mu metal, cause
I don't see anything alarming there?

It could be there to enclose the rupthure if the caps
explosed I guess ? Who knows.

Jamie