The Hewlett-Packard HP-35 was the first "real" pocket scientific calculator. Produced from 1972 to 1975, it cost a hefty $395, later reduced to $295 when the HP-45 came out.
It looks so Spartan today, with very little in the way of shifting to do multiple duty for the keys. It also brought the innovative -- and entirely logical, dammit! -- RPN Reverse Polish Notation to the small calculator world. RPN versus Algebraic would divide the scientific and business calculator worlds for a decade.
And HP calculators were built to last. If you have an HP-35 and its battery charger, it will probably still work on your desk, though I don't know that you can still get the NiCad battery packs for portable use. (There are links in the HP Museum article above to sources for repairing old battery packs.)
A Modern Update
We've seen so many calculator lines come and go over thirty-five years, that I have to give a hand to Hewlett-Packard for recognizing the importance of the HP-35 and coming out with a contemporary scientific calculator in its honor -- the HP-35s. Put the HP-35s up against even an HP-48GX and you'll realize that you've come home. With thirty years of HP calculator ownership myself, after inserting the batteries, I was up and running with the HP-35s without any difficulty, reveling in how some simple operations, like setting up FIX_4 notation, were still relatively simple, despite this age of calculator menus and oodles of features.
Me like it.
Confessions of an HP Calculator Nut
My, I do love my HP calculators. I own at least an HP-65 (bought 2/14/1977), HP-15C, HP-32S, HP-22S, HP-20S (for Mrs. Dr. Phil), HP-48GX (office), HP-48G (home), an HP-12C Twenty-Fifth Anniversary Model and now the HP-35s. They all work, and I use them for different tasks sometimes, because they all do things differently.