Had you mentioned the box and the "PJ" in the query message in the Paperweights forum, we could have told you that it stands for Paul Jokelson, the American importer of Paul Ysart items for many years.
Also, for clarification, the paperweight reference No. is solely that within Frank Andrew's "ysartglass" web pages. It is not an official reference number by Paul Ysart or any of the companies he worked at.
If you choose to update your listing with these details, please add a mention of the GMB as the source of this information.
Good luck with the sale.