NavList:

You can waggle the pictures up and down within the frame.  Doing so shows much more 8 lead rainbow ribbon cable and what looks like a SCART connector. The gearing from the micrometer is only 1:1, so the round things are probably incremental rotary encoders, one for the arm and one for the micrometer drum.  Early Acorn PCs didn’t become available until around 1980, so notwithstanding the date on the calbration certificate, this device might be more 80s than 70s.  I visited the Admiralty Compass Observatory at Teddington around 1980 to swap a Watts Datum Compass and was invited to have a look around while I was there.  There were all sorts of interesting experiments going on in the labs.  This looks like an experiment to digitise the arm and micrometer drum measurement for processing within an early PC, possibly a BBC or Acorn Atom. The OOW might use the Admiralty Pattern sextant as per normal, but the output from the encoders would be fed to an incremental encoder interface to give absolute position and then to a PC.  Then anything might be possible depending upon the programme:  instant observations; observations when a button was pressed; or continuous averaging.  Timing would have been no problem once set in the PC, and sight calculation within the computer might also have been intended.  DaveP