From your very link, 1983's GNU Project was not the first piece of Open Source software.
From your link: The first example of free and open-source software is believed to be the A-2 system, developed at the UNIVAC division of Remington Rand in 1953
> Software was not considered copyrightable before the 1974 US Commission on New Technological Uses of Copyrighted Works (CONTU) decided that "computer programs, to the extent that they embody an author's original creation, are proper subject matter of copyright"
FOSS before 1974 looks.. funny. It existed! But it did not look like the modern FOSS movement.
Even post 1974 and pre-GNU, FOSS-ish text editors and such existed. This was still the era when licenses were often non-standard and frequently did not exist. Handing your friend a copy of a program was the norm, regardless the actual legal situation (which itself was probably vague and unspecified).
From your link: The first example of free and open-source software is believed to be the A-2 system, developed at the UNIVAC division of Remington Rand in 1953