Looking for help with hallmark

I found this ring in my dad’s jewelry the other day. I had forgotten about it, but I do remember him wearing it in the 70s. He probably stopped wearing it due to its poor condition-the chip inlay mostly has fallen out. The mark in an offset SD with a diagonal line under it. My research indicates Singer-Dixon, maybe? Does anyone recognize the mark or can you tell me anything about it? He lived in San Diego when I was a kid, while he was getting his PhD, so it is not inconceivable that he travelled to AZ to sightsee or something. Is there any way to have the chip inlay redone? Is that a thing? I love the design and it has nostalgia for me, I was sad to see it in such poor condition.

https://www.art-amerindien.com/hallmarks/s-american-indian-jewelry-marks.htm

no match here

@Steve that’s where I looked and found nothing.

I thought it was Singer-Dodge family shop. But I could be wrong.