Appears to be very similar to the earlier and overlapping use of "Mailgram" by Western Union:
Many modern facets of postal history take a while to catch on, often 50 years or more ... note the Prexies and the Liberty series. Permit mail, even permits many decades older still have only a small following. Part of that is the lack of readily available information to the collector, catalogs, lists, etc. Lacking stamps, and often postmarks, it is unlikely modern permits will ever get much following, regardless of their low survival rate.