Sterling silver is generally 92.5% silver and the rest is mixed up of other metals, this is due to silvers soft state it would just bend and dent and be totally undesirable so they mix it with other metals to give it strength. I don't know why it would have .99 on it, take it to a coin store or a jeweler and just ask them politely if they can shed some light on it for you if they have the time!
some rare collectable will get a good price but most of the time they are plated or damaged or incomplete so you get a fraction of scrap value at best. Honestly I would use it it will boost your health which is the most important investment. silver is an matural anti-parisitic, anti-microbal,anti fungal, anti-viral metal used in water purifiers too. Thats why back in the day rich kids were "born with a silver spoon in there mouth" for the health benefits, and people would put a silver coin in milk to slow it from spoiling. the you may get alot more from using it that what most places will give you look up colloidal silver and bob beck.