http://www.britishmuseum.org/explore/highlights/highlight_objects/pe_mla/s/silver_disc_brooch_of_%c3%a6dwen.aspx
The problem is that the original fitting was lost in antiquity and it is very difficult to find images of the back of any disk brooches, let alone something of a similar nature.
It would appear that the fitting was part of, or attached to the silver strip which was then riveted to the brooch with the boss pins. This may possibly explain why it has broken in the way it has, if the brooch was forcibly removed. Interestingly the brooch was inscribed with a curse against thieves and I doubt that it's original owner would have treated it so roughly.
It was found in a hoard and I've often wondered if it's new owner came to a sticky end...

The original brooch has always struck me as having been made by someone slightly out of his depth. The concept is good but the execution is slightly crude.
I actually prefer to make derivative items, in the style of originals but not exact copies. This avoids the common problem that we see in re-enactment of ten people all wearing the same originally unique object.
My intention in this case is to refine the design a little and improve on the craftsmanship as best I can, while still using the same techniques. As if this were a practice piece and the jeweller went on to produce a better result on his next try.
From one point of view this gives me some latitude in how I make the fitting, on the other hand I want the result to have some provenance and credibility at least.
My GoogleFoo is letting me down a bit here. Does anyone know where I might find some more information on such fittings?