PD on a ring typically signifies palladium. Your ring could be palladium-plated if it's PD published about it. HK is really a trademark that stands for Harriet Kelsall. They design and style and supply jewellery all around the UK also to other destinations at the same time.  Can “JP” also https://collinjasiy.techionblog.com/37142654/jp88-no-further-a-mystery