Although the A12 bubble sextant (strictly speaking, an octant) was a good deal simpler than most other bubble sextants, there are still plenty of pitfalls for the inexperienced. In my latest restoration manual I explain the structure of the instrument and how to reduce it to its sub assemblies. I then explain how to service each sub-assembly and put them together again, followed by a section on collimation and adjustment. I reproduce the instructions for use chapter from the original manual as an appendix. There are over 50 colour photographs and diagrams in a total of 57 pages.
If you would like to buy this manual, it is available as a down load for US$18.50 or for $4 extra, I can post it to you on a CDROM. Make payment through PayPal, using my e-mail address as the payee : nzengineernz@gmail.com or Contact me.
The Nautical Sextant 
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Excellent book! Both the A12 and the A10/A are must haves for anyone with an interest in use of,or rebulding anituque aircraft sextants. Books are full of easy to understand instructions. Simply put, you don’t have to be an engineer to read and understand how to refurbish these old works of art.