This is a box sextant signed Banks London, which is Robert Banks who was better known for his microscopes, and his instruments were signed either Banks or Bancks. The company later became Bancks & Son in 1820 and made microscopes and telescopes for Charles Darwin. This is almost certainly one of the earliest designs of box sextants as there is a very similar one at the Royal Greenwich museum, made by William Fraser around 1790.
The design is quite different from the later box sextants one sees for sale, and is in working order with the mirrors in good clean condition. T he arm when moved with the small button, does act directly on the internal mirrors to produce images corresponding to the different angle selected on the vernier scale. It also has an internal sliding sun filter unusually operated by a crescent shaped slide on the face plate as well as one on the sliding shutter.
I can see that 2 of the 3 side screws securing the face plate to the main body are missing but this doesn't affect the operation of the sextant, and probably easy to replace. Nevertheless, this is one of the first box sextants made and very rare, so it will be one for a collector. It measures 70mm in diameter and weighs 280grams.Please study the photos as they are part of the description, and I will be happy to answer any questions if I can. This item is in the category "Antiques\Science/Medicine\Scientific Instruments". The seller is "nicke11" and is located in this country: GB. This item can be shipped worldwide.