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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.
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!