I'd say the D305 Jackie Robinson. The D302 set appears to have been released over a few years. See here:
http://www.net54baseball.com/showthread.php?t=171169
And "1948" Leaf is known to be a 1949 issue. Not that 1948 would have cut it anyway. D302, at least with the rounded corners, is from 1947. Is it mainstream? I'd say so.