Mysteries just have to be solved so I just had to find more about Elizabeth Bailey, widow at marriage to Richard Mason.
Going by her age at death, she would have been born about 1749. As the family seems to have been based in Lymington, I looked at the baptisms for all Elizabeths and Bettys in Lymington between 1748 and 1750. The one who stands out is:
Elizabeth Serrel, bapt 10 April 1749, daughter of John Serrel.
There was a marriage of a Joseph Bailey to Elizabeth Serrell in Lymington on 4 June 1767. there is also a burial entry in the register for a Joseph Bailey, aged 42 (b abt 1739) on 31 May 1781 in Lymington. This would tie in nicely with the marriage of Elizabeth Bailey to Richard Mason on 13 July 1783. I've had a look at baptisms but I can't see any children being born to Joseph and Elizabeth Bailey (or variations of the surname) in Lymington.
Just to add a bit of flesh to the bones, the Hampshire Chronicle of Monday 3 March 1828 had the following announcement about the death of Mary Jane Mason's brother Richard (1794 - 1828):
Lymington
Died on Monday, in the 35th year of his age, after a long and painful illness, Mr Richard Mason, confectioner of this town, leaving a wife and two children.
The Hampshire Advertiser of Saturday 7 November 1835 reported the death of his widow:
Lymington
Died, on Tuesday last, aged 42 years, Jane, relict of the late Mt Richard Mason, confectioner of this town.