@Tailspin:
Your correction suggestion of June 9, 2015, apparently based on Praguefrank's Bob Atcher page, was unsubstantiated. As commented here before, Columbia's 20,000 C&W series did not start before May 1948. All records with numbers lower than 20426 were reissues. So Columbia 37991 and Columbia 20393, against the impression conveyed on Praguefrank's site, were not released at the same time. Columbia 37991 came out in Dec 1947, as suggested by you here, but Columbia 20393 was reissued in 1948, probably in June, see the Columbia ad of June 19, 1948 quoted by LaurenceD, or even later. (Atcher's following single, originally issued on Columbia 38132, is already listed in the ad with its reissue number 20410.)
Putting in my two cents: Billboard magazine of May 1, 1948 (Advance Record Releases, pages 34 & 134) already announced Columbia 20432 by Hoosier Hot Shots, one of the first original issues in the 20000 series, simultaneously with Columbia 38189 by Roy Acuff, apparently the last C&W record in the 38000 series, reissued on 20425 then. So I guess that the original numbers (higher than 20425) started from day one and reissues of records of the 36000/37000/38000 series followed in stages on demand, though with previously allocated lower numbers.
Other low numbers of the 20000 series, however, came notably later, e.g. 20084 - 20087 (records of a Gene Autry album set) which were released on May 12, 1948, according to W.B.lbl again.
W.B.lbl, are you able to enlighten us? Did the 20000 series actually start on May 1, 1948? Do you know which numbers were released then? When did the original releases in the series begin? Having checked 78discography.com, it seems that everything before 20426 was reissues.
Exact release date added/edited by W.B.lbl for Columbia 20013, 20051 and (surprisingly) 20377 is "1 May 1948" which may be the day of the series' start.
Could not find a BB listing for this cat#, but neighboring releases suggest mid-1948. Oddly, there's an ad (Jun 19, 1948, p.62) which lists this pairing under "latest releases" but with the old 37991 number.
Which reminds me, does someone here know when the 20000 country series made its debut?