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The Sinclair Oxford 150 belongs to a series of calculators that include the Sinclair Oxford 100, 150, 200, 300 and the Sinclair Oxford Scientific.
This one is a basic calculator with square roots and percentages.
This calculator thinks √-9=3.
A nice write-up on Clive Sinclair and the Pocket calculator can be read on this vintagecalculators.com page (link valid 2024-01-13).
Thank your, Arnoud Helmantel, for finding this one on a flea market!