Nowadays, each year's baseball cards are issued in the spring, around the beginning of the baseball season, and it's easy to assume that it has always been that way. But that does not appear to have been the case with 1948 Bowman baseball cards, the first major set of gum cards to come out following a 7-year hiatus caused by WWII. Contemporary evidence suggests that they were first issued in late summer, around August. That may be why there were only 48 cards in the set, issued in two series; by the time the second series came out, the baseball season was almost over, and there was no point in issuing more.
The leading sports collecting hobby paper in 1948 was The Sports Exchange Trading Post, of which I have a complete run from January 1947 to the last issue in January 1950. I looked through every issue from January 1948 on for any reference to Bowman or any other new baseball cards. The first mention I found was in the August-September 1948 issue (published in September) in the "Fan's Corner" column, in which editor Jack Seifert published news, wants, and offers from readers. A reader named Joseph F. Driscoll from Brooklyn wrote that "gumcards are back with "Play Ball" gum by Bowman, Inc., Philadelphia, Pa., 5 cards in each pack of gum. He is willing to sell some of these..." This means that the cards were out by late August or early September, but they had apparently not been out in early July, when the July issue went to press.
Wirt Gammon, who would later write regularly for many different hobby publications into the 1980s, had a semi-regular column in Sports Exchange Trading Post called "As Gammon Sees 'Em", focused on collecting baseball items (and occasionally items on other sports). His columns in the March, April, and June 1948 listed many new baseball collectibles that Gammon or his readers had encountered, but there is no mention of Bowman or any other new baseball cards (other than some postcards). Gammon did not have columns in the July or August-September issue, but in the October issue he reported that three new sets of collector's cards were out: "The Trading Post's new baseball miniatures" (what we know as W602), "Swell Bubble Gum's set" (R448 Sport Thrills), and "Blony Bubble Gum's" (1948 Bowman baseball, which had an ad for Blony on the back). Note also that Harry Caray had a 1948 baseball guide out!
Finally, here is a brief note, dated September 17, 1948, that Buck Barker wrote to Jake Wise on the back of an Exhibit card of Charlie Gehringer. He asks Wise, "Have you any of the new "Picture Card Play Ball" bubble gum set? I can get you nos 1-36, except #4."
It might seem odd that Barker refers to 1948 Bowmans as "Picture Card Play Ball" cards, just as Seifert had referred to them as "'Play Ball' gum by Bowman, Inc.", since those words do not appear on the cards. But a look at a 1948 Bowman baseball wrapper (from the Old Cardboard site) clears up that mystery:
The fact that Barker only mentions numbers 1-36 suggests that the high numbers (#37-48) may not have been issued yet by September 17, or at least had not yet reached Barker in St. Louis. By the following May, when Barker wrote a two-page catalogue of gum sports card sets for the June 1949 Sports Exchange Trading Post, he knew that 1948 Bowman baseball (which he called "Play Ball Baseball Gum") had 48 cards:
I first posted the above article along with Barker's other SETP columns in this thread:
http://www.net54baseball.com/showthread.php?t=222057, but I'm posting it again here for ease of reference. Notice that Barker (writing no later than May 1949) does not list 1949 Bowman or 1949 Leaf baseball, or indeed any 1949 sets, saying that "I hope there will be more this year".