I came across these things years ago at the local scrap yard, a
favorite prospecting locality. Typically they were on heavy
copper, not steel, were soldered on & easily unsoldered. I actually
sent one to a lab for analysis; to my surprise it turned out to be
Pure silver is the most electrically conductive metal and is used
for relay and other switch contacts. But it is soft and contact wear
from arc erosion and mechanical wiping is a problem, so there have
been a whole range of alloys developed to increase the contact life.
Cadmimum, copper, zinc oxide, tin oxide, zirconium oxide, nickel,
gold, palladium have all been used in silver alloys for electrical
contacts. So unless you do lab tests it is hard to know what exactly
the contact material is.
James Binnion Metal Arts