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HP-12c Platinum
(feddit.uk)
HP's most accurate financial calculator, oddly enough, and despite only returning the ceiling of solve-for-n.
My dad was a surveyor almost my entire life, he always had this (or a very similar model) in his pocket or on his desk.
As a surveyor, he probably had an 11c or 15c (the 12c is the financial version of the Voyager series).
Aside: Swissmicro sells a series of voyager clones with modern hardware:
https://www.swissmicros.com/product/dm15l
More info on the original HP calcs:
https://en.wikipedia.org/wiki/Hewlett-Packard_Voyager_series
I looked at an image of the 11c, that was it for sure.
That's lovely - these calculators are very 'dad' style to me also (though my dad had a 70s style Casio). There's something about the voyager HP calculators with their landscape format - they are really satisfying to use, I can see why he kept it on him!