It means £I am starting to write"I am starting to write in a year which I am not specifying which is 17xx", ie at some point in the 18th century.
It was common to omit details, such as calling a person "P----" instead of the full name. It's interesting to note that in Edgar Allen Poe's The Murders in the Rue Morgue, which was published in 1841, we see the same form: "Residing in Paris during the spring and part of the summer of 18--". Here's a whole question about that: https://english.stackexchange.com/questions/9479/why-in-old-books-are-dates-often-given-with-the-years-redacted
PS. As Treasure Island is an extremely well-known book, it would normally be "It is from the novel Treasure Island" and not "a novel ..."