From https://stjarnhimlen.se/comp/time.html:

"International Atomic Time (Temps Atomique International = TAI) is defined as the weighted average of the time kept by about 200 atomic clocks in over 50 national laboratories worldwide. TAI-UT1 was approximately 0 on 1958 Jan 1."

i.e. 10 leap seconds had occurred between 1958 and 1972.