This is a Barrett Sub‑Normal Vision Monocular and Testing Set, an optometric aid for assessing and prescribing magnification for patients with low vision, made in mid‑20th‑century. 
 
What it was used for:
The set was designed by C. D. Barrett and supplied by the London Optical Company as a “sub‑normal vision monocular,” essentially a trial monocular system to determine what magnification helps patients whose vision cannot be corrected fully with ordinary spectacles.​ The black discs with numbered tags and apertures, together with the cylindrical monocular bodies, allowed the practitioner to test different powers and fields, often in conjunction with a trial frame, to optimise reading or distance vision in one eye for people with partial sight.​
 
Age and maker
Museum catalogues describe the Barrett Sub‑Normal Vision Monocular and Testing Set as a 20th‑century optometric instrument, specifically placing surviving examples and the printed “Directions for Use” leaflet in the mid‑20th century.​ The London Optical Company and associated firms such as Levers Optical Co. supplied these aids mainly in the post‑war decades when structured low‑vision services were developing, dating this set to around the 1940's.
 
This set is offered as a collectable curiosity only and not for practical use.
 
The case measures 16 x 7.5cm (6.3" x 3").
 
An example is held in the Science Museum Collection in London which can be viewed through the link below:
 
https://collection.sciencemuseumgroup.org.uk/objects/co152375/barrett-monocular-and-testing-set