Mystery of an old Harley Davidson ring

Recently I bought this ring in North Africa and have more question than answers about it!
The ring has already been worn, but I'm not sure it's actually old enough (though, I'm not an expert), and it's silver with turquoise. I liked it at first sight, now I wear the ring on daily basis and don't feel like taking it off my finger. That's why I really want to understand who, when and where could wear it before.
The only thing I've found out is that the symbol, an eagle with lightnings of this form, is an old emblem of Harley Davidson. Besides, the ring has a "925" silver mark inside.
The hallmark is situated in the middle of the outside of the ring and seems completely incomprehensible to me. Sometimes I see letters, like "IS", sometimes - number "75" and sometimes even a picture (if you look in the middle of the hallmark, at the lighter parts of it). Searching in internet didn't give any result...
Does anyone have any idea of this hallmark or anything else about this ring? I really would like to know what I wear with such pleasure!