Donatello Chellini silver roundel

I have had this silver roundel for some time and would dearly like as estimation of its value. The plate in total weighs 49oz butit says ‘filled’ on the back which I presume to mean in is filled with plaster. I seem to remember the silver weight being around 30oz. This is a rare item, no.656 of only 750 casts taken directly from the original.
The item is in great condition and still has the original documents and box, has anyone seen one before?
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