A pre-electronic pocket calculator. This slide rule was invented and produced by Otis Carter Formby King (1876-1944), a grocer and engineer from London, England. Its cylindrical shape and helical (spiral) scales make it unusual as a slide rule. Known as Otis King's Patent Calculator, it was used initially is business and was sold from 1922 up to the 1970's.
This one, I believe dates from the 1950’s/60’s and comes complete with its leather case.
It measures approx 15cm when closed, opening up to approx 26cm when fully opened. The calculator is excellent condition, the stitching has come undone on the top of the leather case.
The scale is 423 which is No: 44 and the Model is No: X4377.
This one does not come with instructions but the full text of the instructions is available online: https://archive.org/stream/otiskingcalculator/otiskingcalculatormanual_djvu.txt
There is also a YouTube video which tells you how to use it.
A fascinating gift for a maths lover!