In 'Maltese Cross Cancellations' (1971) Alcock & Holland notes this office as the source of a 'violet' MX in August 1843. This item may well be the cover to which they referred.
In 1975 Robson Lowe also referred to a 'violet-blue' MX from this office in 1843 stating: "Only noted on one cover; the ink on the cover shows a tinge of green whlie the cancellation is distinctly violet-blue on the stamp".
S.A.Stafford was a contemporary & major client of both the above Philatelists.
Interestingly his album page described the colour of this cross as 'Lilac'. One wonders if he was attempting to describe the unique subtlety of this pastel shade.