The earliest known version of "Oregon Trail", dated 1975, in HP-BASIC: https://www.filfre.net/misc/oregon1975.bas

The very first version, written in 1971, appears to be lost to history; the authors deleted it from the computers where they wrote it, and only kept a printout of the code, which has since gone missing. The 1975 version is based on that printout of the 1971 copy, though, and appears to be pretty close to the original code.