William Emerson also touched on some of the issues behind this in his article about 1948-pattern chevrons here (PDF link):
http://emersoninsignia.net/files/Download/9TwoInch.pdfWith Army branch insignia worn as brass on collars, and so on, adding the branch insignia to the existing chevron rank insignia would seem to have been largely redundant, unless the intention was also to standardise to generic pins for all grades of enlisted soldiers and to have specialty marks only appearing on the grade insignia, thus simplifying the rest of the badges and therefore offset some of the cost.
Bear in mind that enlisted collar brass at the time was broadly "regimental" based on assignment (and often incorporating unit numbers and letters in earlier times), so a cook and a clerk/typist in an infantry unit would wear the same brass as a rifleman. Medical personnel were an exception.
The other interesting thing about that excerpt from the Army Information Digest is that the previous page displays the then "new" (1947-pattern) Warrant Officer insignia, which were approved but, ultimately, never implemented.
Yes, the Institute of Heraldry website has illustrated those insignia for a long time, but without noting that they would have been worn oriented 90 degrees from lieutenants/captains bars.
Justin