Well, you've helped me! I have a ring with the exact same 18ct, Pd in trapezium shape and a T in a circle. Mine is a bezel set diamond trilogy and definitely not a size T. I wonder if anyone could tell me any more about the history Thomas Jewellers? I can't find much info about them in any of my Australian jewellers books or online. They were established in Ballarat 1896 by founder James Thomas and then presumably handed down through the generations. I will have a look at the PROV but thought I'd see if little_closet has any local history they could share?