Vintage authentic 10ct Opal Brooch

I've never seen anything quite like this authentic, it was my Grandmothers and my mom has asked me to sell a few things of hers for her.
I researched the markings for hours and could locate anything like them, so if anyone recognizes them or has any information on them, it would be appreciated.
I'm fairly certain it's 10 ct gold based on the marking in the last photo.
There is no damage to the pin and it is still in working condition.
I'm pretty sure that the bar pin holds a crystal, the opal is spectacular and in mint condition. It really has a lot of fire to it. Hard to capture in photos.