Here's the blurb from the site:
About British Postal Service Appointment Books, 1737-1969
These British Post Office appointment books are indexes to the Postmaster General’s minute books. They show the point when a person began working for the Post Office or started at a new position within the Post Office. The books were kept from 1831 until 1969, when they stopped being kept because of legislation and human resource procedures. Some records contain the job the worker was appointed to and the British Postal Museum’s website has the abbreviations in their Family History Guide. The index contains:
Date of appointment
I'm not sure how the records have entries back to 1737 if the collection started in 1831, but I have no reason to doubt it is the case - if your ancestor worked for the Post Office, this should be useful!