No luck on Rhett's list of differences between red and blue backs, but I did find this ad from January 1928 issue of Baseball Magazine, which does seem to indicate that these were still being distributed four years or so after the original release, from a company (I assume headed by George Groves) in the Greater Los Angeles area:
waltermails1928ad.jpg
This ad was shown on this thread:
https://www.net54baseball.com/showthread.php?t=236679
Hopefully neither rain nor snow nor sleet nor hail shall keep you all from delivering the Mails (info/card examples).
Brian