After quoting the page from the Baronetage that includes the entry for the Elliots, Austen writes:

Precisely such had the paragraph originally stood from the printer's hands; but Sir Walter had improved it by adding, for the information of himself and his family, these words, after the date of Mary's birth -- "Married, December 16, 1810, Charles, son and heir of Charles Musgrove, Esq. of Uppercross, in the county of Somerset," and by inserting most accurately the day of the month on which he had lost his wife.

This reminded me of the custom of recording such important family events in a family Bible. Was this a custom in England at this time? If so, then it says a lot about Sir Walter that he's recording such things in the Baronetage, not a Bible. But even if recording events in a Bible wasn't a custom, it still says a lot about him.