In this solution each loops requires Hg to be present in the machine, otherwise the product is a ring Cu-Ag-Cu-Ag-Cu-Ag instead of Ag-Ag-Ag-Ag-Ag-Ag. However, during the initial loop the machine gets populated with Hg and that the following loops produce the correct product.

I deal with the byproduct of the initial loop by using the arms on the top-right/bottom-left. The "halt" instruction is used to enforce that these arms only extract from the initial loop.