it is the Registration number of the boot - prefix by pl (for Peel) and you can look it up
- started as the appendix in 1792 report puts it An Account of the number of Ships and Vessels belonging to the Port of Douglas, Isle of Man, with the Names and Tonnage, which have been registered in pursuance of an Act of Parliament passed in the 26th year of his present Majesty [ie between Oct 1785 to Oct 1786] , entitled, "An Act for the further Increase and Encouragement of Shipping and Navigation," distinguishing such as are British built, such as are foreign built, and such as are prizes made free; also distinguishing which of the said ships have been lost, broken up, or otherwise destroyed or condemned as unfit for further service;

The more modern register is now computerised but as of yet I have not seen it on line