Hi Edgar,
Welcome to the forum.
Arg 900 could be short for Argentum, the Latin word for silver and 900 could be the standard, fineness, 90% pure silver used.
The problem however Arg could also be short for Argentann also known as Alpacca, Neusilber, and Nickel silver, all which contains no real silver 0% pure silver used.
See;
http://en.wikipedia.org/wiki/Nickel_silverI personally should advise you to have the items tested for silver. Your local jewellery shop could advise you.
Regards,
Oel