The CASIO FX-602p series
Find-it on ebay.com -
 

 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.

 Programs on geocities.jp
With FA1 device for back up to tape.