Welcome to the GMB, Liz.
This is an uncommon Sowerby tableware pattern, but is shown on page 50 of the Sowerby pattern book XI of 1885 as a sugar bowl, and a covered/lidded example is shown on page 59.
The pattern is shown as being from a registered design, so when you say it has the Sowerby ‘stamp’, does it have an embossed Sowerby peacock head trademark or a diamond-shaped registry date mark (or both)?
I don’t have a reference photo for this pattern so I don’t know the precise registration date for the design, but I would expect it to be some time in 1879 or very early in 1880.
If there is a diamond-shaped registry date mark could you let me have more details of the letters and numbers in the internal angles of the diamond , please? - preferably beginning with the letter or number at the 12 o’clock position [under the III in a ring] and working clockwise around the Rd mark in the centre of the diamond?
From those details I can get the full date of registration (including the Parcel number), and the registered design number(s) in the parcel.
Fred.