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Yes, the 1950s. Just.
The US and the Commonwealth agreed on a new standard for the inch in 1958, and it came into force on 1st January 1959, when the yard was defined as 0.9144m exactly and since there are 36 inches in a yard, that made an inch exactly 2.54cm.
Before that, the Imperial inch was a little smaller (approx. 2.539988cm, but not defined in terms of the metre), and the US inch was a little larger (exactly 2.54000508cm, and defined in terms of the metre, with 1 metre being precisely 39.37 inches).
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