I used to have some tables for brownbuilt purlins but can't find them.
It's an 8" deep purlin, the 12 likely refers to 1.15mm base metal thickness metal. This is based off current 200/12 purlins being made from 1.15mm bmt metal sheet. So I'd expect if the purlin has stiffeners then it's performance might be similar to a current 200/12 purlin. If there are no stiffeners you might need to look at it from first principles.