Figure it this way: Bing Crosby Enterprises imported some of the first magnetic media recorders to the U.S. in 1946. By 1947 Crosby had commissioned Ampex to build their own version and by the Summer of `47 was using it for his radio show, pre-recording for the 1947-`48 season.
The big labels took to it right off, with Columbia and RCA leading the pack, both of whom had their first magnetic sessions in either late 1948 or early 1949 as sources differ.
Sessions by the smaller labels e.g. Mercury were done on disc as late as 1952 or 1953 meaning the big studios were already doing the first twin-track recordings (2-track mono - vocals on one orchestra on the other) at the same time as some of the smaller studios were still transitioning from single-cut 16-inch or 17-1/4 -inch 33 RPM lacquers which were then played back directly on one lathe and sent directly to another to cut the 78 RPM 12-inch mastering blank (for 10-inch discs and 14-inch for 12-inch discs) from which the pressings would then be made.