How old is this ring? J Bros 2 tone band 18bkt

by Lisa
(Redding, Ca)

This ring was purchased at an estate jewelry store. I have not been able to find anything like this on the internet. The ring is two tone 18k gold marked on the inside with J BROS and 18bKt. The engraving says 4-12-16?

Curious who the maker's mark belongs to and the reason for the b and t in the 18k. Why is the ring yellow gold inside and white gold on the exterior? Thank you for any help!