Wiktionary
a. Usually used before century to indicate the middle part of the 20th Century, which could refer to events in the 1940s, the 1950s or the 1960s. However, in architecture, preservation, and real estate, it is commonly being used to refer to the period after World War II to the mid-1970s.
Usage examples of "mid-twentieth".
We’re not like those idiots back in the mid-twentieth century who thought autistics were just ignoring people.
The reigning Kronian model of what had taken place traced back to heretical challenges to orthodox astronomy that had first been proposed in the mid-twentieth century and held that after its violent birth, Venus had careened about the Solar System as a loose cannon, disturbing the orbits of both Earth and Mars and eventually circularizing its own orbit to become the planetary body familiar in modern times.
Ruskinson triumphantly announced that Luscus had taken the phrase from obsolete slang, current in the mid-twentieth century.
If you lived in a technology like that of the mid-twentieth century, you were there to see the vacuum tube displaced by the transistor and that by the tunnel diode, while in one ten-year period the artificial satellite moved from the area of laughable fantasy to a hunk of hardware broadcasting signals from the other side of the sun.