Bulletin Horaire was the regular publication of the Bureau International de l'Heure (BIH, International Time Bureau). Nicolas Stoyko served at and ran BIH for four decades, and his wife Anna Stoyko also served there. They retired in 1965, and they wrote monographs for their final issues of the Bulletin. See here for more views of their summary data.
Anna Stoyko reviewed the 42 years of data from BIH and produced plots showing the improvement in accuracy and precision of the time services of participating observatories from 1922 to 1963. Those plots are reproduced here.
Note: these values show the differences after removing persistent systematic errors between observatories which were largely caused by inconsistencies in the conventional longitudes that each observatory used when computing time. Systematic differences of 0.03 s or more in the UT reported by observatories persisted until the late 1960s when satellite data prompted the creation of worldwide geodetic systems, and by that point all broadcast radio time signals were already providing time based on atomic chronometers rather than on direct astronomical observations.