Captain America Shield - Circular patterns on rear?

[BeLikeWater]

I know there's a million threads on this prop, so apologies for the additional one.

With all the reproductions being created, I see no shields that have the circular patterns on the back. Why is this so difficult to obtain? What's required to do it properly?

Thanks!

 

[Fly4v]

The steel shields are too thin for anything but a line, the aluminums are more expensive to have the inside spun but it can be done and there are some out there. All the airsoft shields have the interior pattern.

 

[Kevin Gossett]

Like this?

 

[BeLikeWater]

Like this?

http://i60.photobucket.com/albums/h33/wkevingossett/20140506_111709_zpspflrgvng.jpg

Yes, exactly!!! How is this done? I mean, how would something like this be done on an aluminum shield? Where could I have it done and how much additional would it cost?

 

[Kevin Gossett]

It was done with the initial spinning. Groves were cut on the front, and then the back.