HP-15C — Wikipédia

Retro Gadget: Cold-war Era Programmable Calculator Takeapart, MK

The Creator (novelette) - Wikiwand

HP-22S

My grandpa's 1965(ish) HP 21 Calculator : r/BuyItForLife

Calculator for an electronic engineer

HP-28 series - Wikipedia

HP-41C compared to Sharp PC-1211 (Valentin Albillo) - Hardware

HP-42S - Wikiwand

HP-15 Simulation

HP 25 computing prime factors of 9,999,999,971 : r/retrobattlestations