not a clue
"760" marked on any silver means that the object contains 76% silver (and 24% other metals).
99 on silver means 99% (or more) silver, or pure silver. Sterling silver is marked 925 (or 92.5%).
No. Jewelry stamped 925, .925 or "sterling" is genuine sterling, not silverplate.
925 = silver, gold color = gold plating
"966 sterling" does not have a standard meaning in relation to silver or sterling silver. Typically, sterling silver is marked with "925" to indicate its purity level of 92.5% silver. If you have a piece marked as "966 sterling," it may be a manufacturer's unique stamp or a mis-marking.
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If it's .550, not just 550 it is referring to the percent (.550x100=55%) of silver the product is made from.
This means that the item is 80% silver and 20% alloy. Sterling silver is 92.5% Silver and 7.5% alloy. Many silver items produced in Mexico are marked 800 or 80.0.
Sterling silver is marked .925; fine silver is marked .999. It appears 3645 may be a pattern or product number.
Silver is marked on the 25th year of marriage.
I don't know, but I have what looks like a very old sterling silver pin. It is only marked SGW. Wish I knew!
928 is likely the purity of the silver that the piece contains in parts per thousand. Most American silver is marked 925, German silver 800 or 830, etc.