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Al Stewart. You were asking about your Heath sextant.

I agree with Bill Morris’ conclusion that your Heath sextant is pre 1969.  This is only the date on its most recent calibration certificate.  Heath was merged with WF Stanley & Co in 1937, and you can’t find much about WF Stanley & Co after 1960 (https://www.gracesguide.co.uk/W._F._Stanley_and_Co) .  A previous owner must have needed it overhauling or recalibrating in 1969.  Heath clearly wasn’t issuing certificates by then, and the National Physical Laboratory stopped calibrating sextants in 1947 (https://www.cambridge.org/core/journals/journal-of-navigation/article/div-classtitlethe-admiralty-compass-observatorydiv/A2858BB54D3A0E387B0EA4C04779E80E). The choice in the UK would have been to take the sextant to the Admiralty Compass Observatory, B Cooke & Son, Hull, or Kelvin Hughes.  Kelvin Hughes must have been chosen.  The Perspex over the certificate is typical of post WW2 Kelvin Hughes sextants, but I can’t imagine Hughes using what look like aluminium Phillip’s or ‘Posidrive’ screws. As late as 1961, they were using round headed, slotted brass screws to retain the Perspex.

Regarding adjustments, you can’t be more than 10 miles from the sea.  Why not just try your sextant as is?  You don’t even need to do any calculations if you use Frank’s GPS Anti-Spoof App.  If you get satisfactory results for your requirements, forget further adjustment.  I’m always wary of making things worse, but that’s probably just me. DaveP