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Thanks,  David, Drago, Tom

David, i managed to get the nut loose. The Machine screw is actually pinned to the frame. I can clearly see the pin. The nut appears to be the same on both the horizon and index shades.

There appears to be black variant of the Mate sextant which is as you describe. 

I have since found another picture of a Mate sextant and it appears to have the identical assembly and nuts. I have attached the picture and you can clearly see the same nuts.  

I bought it for about 15 dollars and it was in a mess. I placed it in the dishwasher and it looks as good as new now. I used  Deoxit on the battery case. light switch and  the globe socket and everything works. The telescope eye piece with no filtering was missing so the shades are redundent since I have the eyepiece with the shade. Despite having no case and in a very filthy condition  the mirros and mirrors are all perfectly fine. Its been one of easiest sextants to adjust.

Drago, thee tool shop suggested that I modify a screwdriver which i will do. Woodworking shops sells the driver because the nuts are used on the old wooden saw handles. I just found out this today. I will make one because the woodshop version is an expensive exotic tool with rare wood handles.

I went to a specialist nut shop and they referred to these nuts as "Slotted Round Nuts (DIN 546)" Special order.

The sextant is  built like a brick outhouse. I will try and find a collimator  sometime  to check its accuracy.

Thanks all for the help.

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