General purpose calculator with capability for performing yield-to-maturity of a bond calculation
The oldest grave in the corpus: a Hewlett-Packard calculator patent from 1974, for a machine that could compute a bond's yield to maturity at the push of a button — a genuinely novel thing to put in someone's hands, fifty years ago.
Assignee: Hewlett-Packard Company, L.P. · Filed 1974-10-10 · Granted 1976-03-23 · Term end (est.) 1994-10-10
US3946218 was filed October 10, 1974 and granted March 23, 1976 — describing a general-purpose calculator built with the specific capability of computing a bond's yield to maturity. Before this class of device, that calculation meant a bond table, a slide rule, or a mainframe — not something that fit in a briefcase.
It's the oldest patent in this corpus by grant date, and its term ran out under the pre-1995 rule (17 years from grant) on October 10, 1994 — over three decades ago. Yield-to-maturity on a handheld calculator has been unrestricted, ordinary technology for most of the people alive today.
It's a useful anchor for what 'free' looks like at fifty years' distance: total, unremarked, and completely absorbed into what a calculator is simply expected to do.