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When I started collecting Casio calculators, I did not think going back so far until the 70's, at the time the calculators had Nixies or VFD screens.
And then, under the evil and friendly influence of gégé, met on silicium forum, I started looking for them.
I love the fx-1, 2 and 3, the old models AL-1000 or 2000, the mini series, the pocket series and the CQ models.
My favorites are the fx-201p series, programmable in Fortran, including the pro FX-1, the only Casio model with backup programs on magnetic cards.