I find the 21 JUN 1948 date comes from many sources and my be correct.

Columbia released many LPs at the same exact time giving the an entire catalogue to choose from.

The fact that the Sinatra LP has the first number in that series and the Bruno Walter disc has the lowest number in their Classic catalogue means that the numbers were chosen that way.

The history of recording is really fascinating.

33 1/3 rpm and "microgroove" stylus were both tried by other companies many years earlier, but the combination of the two, as done by Columbia in 1948, stuck.