I found a similar ring about 30 years ago at an 1890's school site. It was also marked "SOLID GOLD". I took it to a jeweler who said that they use to mark jewelry that way. Just as you stated The National Gold And Silver Stamping Act of 1906 was passed to eliminate those type of markings so that the purity of gold and silver items would be known to the consumer. Mine turned out to be less than 10K but it did have a gold color to it.This would date your ring to pre 1906. It's a nice old ring no matter what the alloy. Good hunting!