I also have a copy of this paperweight and the certificate that came with it. Back in the early 1970's Bernard and Dorothy Broughton opened their home in the Village of St Tudy, Cornwall, to visitors and displayed their paperweight collection so the ST stands for St Tudy (Paperweight Centre). The collection was later moved to Yelverton, where it remains today, and other paperweights were made with a Y 81 inpressed in the base by Liskeard Glass which is now Merlin Glass owned by LIam Carey - still in Liskeard.
Hope this helps
Dave