^:(1) In "The Physical Basis of the Leap Second", by D D McCarthy, C Hackman and R A Nelson, in Astronomical Journal, vol. 136 (2008), pages 1906–1908, it is stated (page 1908), that "the SI second is equivalent to an older measure of the second of UT1, which was too small to start with and further, as the duration of the UT1 second increases, the discrepancy widens."
(2) In the late 1950s, the caesium standard was used to measure both the current mean length of the second of mean solar time (UT2)(measured over a period of about a year) (result: 9192631830 cycles, probable error not stated), and also the second of ephemeris time (ET) (measured over a period of about three years) (result:9192631770 ± 20 cycles): see "Time Scales", by L. Essen, in Metrologia, vol. (1968), pp. 161–165, on p. 162. As is well known, the 9192631770 figure was chosen for the SI second. L Essen in the same 1968 article (p.162) stated that this "seemed reasonable in view of the variations in UT2".
^WP7D - Status of Coordinated Universal Time (UTC) study in ITU-R (Word 2007). International Telecommunication Union – Radiocommunication Sector (ITU-R) Release. 2011-10-04: Pg.2 (Pgs.2) [October 24, 2011]. "To date, the BR received replies from 16 different Member States for the latest survey (out of a total of 192 Member States, 55 of which participate in the formation of UTC) - 13 being in favor of the change, 3 being contrary."