Thanks to information and images provided by the owner of Unwinding the Skein, there appears to have been a third figure on the right of the canvas, but which has been cut off. The edge of the skirts of a third dress appears to be just visible.
The owner also notes how the right-hand figure on the existing work appears to have been wearing a hat, but that this has been painted over. The signature is confirmed as ‘ALS’, not ‘AS’ as suggested on the retailer’s web site.
The image is a slight puzzle, being in the style of Annie c. 1880-1890, yet of a relatively crude quality, and with the lettering of the ‘ALS’ signature more characteristic of her post-1900 work. Also, the back of the panel has another canvas pasted on, preparation for another painting. (Many thanks to R. W., the owner of the work, for these observations.)
My theory is this was painted late-1880s as a trial piece and never intended for exhibition or sale, but which was later cut down, signed and framed for exhibition/sale purposes.
The New Gallery catalogue for 1888 mentions an untraced work, Parcæ, depicting “three Cornish girls, with skein of wool, one in a bright red dress; half length, nearly life-size.” Perhaps this was a preparatory sketch for that work. [Addition, 11 Apr 2022: no longer the case. An article in The Queen (a newspaper), 5 Mar 1890, describes ‘Parcæ’ as “three fisher girls winding wool at a quayside … against a mass of houses darkly purpling up against an evening sky,” different to the image above. Annie may still have had thoughts about the Parcæ in mind. The parcæ were the three ‘fates’ of Roman mythology, three female deities who determined people’s destinies of men and who are often depicted in art spinning wool or working with yarn.