ताम्रपादयुक्तं रूप्यं रूप्यपादयुक्तं वा सुवर्णं संस्कृतकम् तस्माद् रक्षेत् ॥ कZ_०२.१३.४२ ॥
tāmra-pāda-yuktaṃ rūpyaṃ rūpya-pāda-yuktaṃ vā suvarṇaṃ saṃskṛtakam tasmād rakṣet
Silver fitted with copper feet, or gold fitted with silver feet, is a “worked/treated” (composite) item; therefore it should be safeguarded (carefully controlled against fraud).
Mixed-metal construction can disguise substitution at joints/bases; it is easier to skim precious metal while keeping appearance.