The CASIO FX-602p series

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 These series includes the oldest programmable pockets
 Pocket computer from late 70's
 The FX-5XXp

 First difference visible on the keyboard, the number of available brackets : 5 for the 501p, 10 for the 502p.

 Then the [.] key has a second function [10] to give access to the ten memory registries.

 The rest of the keyboard is identical on both calculators.

 Third difference visible in mode 2 : 130 steps for the 501p, 255 for the 502p.

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