Your description leads me to believe you have the same makers mark or it may be a trademark as the 2 rings I have come across in my antique shops. They have the same features.
The link below is a photo of the ring with the hallmark or trademark symbol please let me know what you think...
The mark is so small! I have been digging online for awhile now and I believe your symbol is HN inside and oval for the company Heng Ngai from Hong Kong but has affiliate company in Irving TX. Hope this helped!