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I have several sextants and soon after acquiring each, I obtain spare mirror adjustment tools. My Cassens & Plath was easy, its a metric 'allen' key readily available at hardware stores. But I had to make my spare Astra IIIB key.  Here is what I did:

Materials:

Printed Circuit Board (PCB)is very inexpensive, available on eBay, Radio Shack (if one still exists). PCBs are thin paper reinforced phenolic resin with a bonded copper layer. They are stiff, strong and can be easily soldered to.

Hobby Shop copper square miniature structural sections. This is hobby shop item and frequently used in making model railroad scenery and structures.

Steps to making Astra IIIB mirror adjustment wrench:

Key 1 - PCB front/back and a section of square copper 'tubing'
Key 2 - detail of copper tubing which forms the tool's socket
Key 3 - PCB and tubing cut to make sextant mirror tool
Key 4 - PCB handle drilled and filed to accept square copper socket
Key 5 - copper socket is soldered to PCB
Key 6 - Factory supplied Astra IIIB made more conspicous by adding a float

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