Around 1975 Greg Deering and his fledgling banjo company did make a bunch of mountain dulcimers. Your might be one of them. What makes you think it's a Deering?
From the web:
"...So, around July 1975, we officially started Deering Banjo Company. That lasted until November 1977. Geoff [Stelling] had bought a shop building and I was renting part of the building from him. I had a crew of about nine people. We were making about 40 banjos a month. And Geoff came to me one day and said, “You shouldn’t be Deering Banjo Company. It should all be Stelling Banjo Works, and you should just be working for me.” And I said, “I’m not going to do that.” The next day, he cut my orders in half. So, I stayed up all night and built an Appalachian dulcimer and, when my crew came to work the next day, I put half my crew to work making dulcimers. I went out, got a bunch of orders for them and just kept on going...."
also:
"...The family-owned Deering Company, which was formed in 1975, has 49 employees. The enterprise has the self-descriptive slogan “The Great American Banjo Company,” with “Proudly Made in the U.S.A.” emblazoned on banjos made there.
Janet Deering is CEO of the company, and during a tour of the manufacturing areas of the facility, she spoke warmly about the deep roots the company has nurtured in East County throughout its history. Her husband, Greg Deering, made his first banjo in an industrial arts class at San Diego State in 1968. The couple initially made banjos and mountain dulcimers in the garage of their home, before moving to a Lemon Grove location. After outgrowing that facility, the company moved to its current factory site in Spring Valley during early 2001. Over its 40-plus years, Deering has built over 100,000 banjos. ..."
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Site Owner
Those irritated by grain of sand best avoid beach.
-Strumelia proverb c.1990