Which Rolling Stone magazine from 1974 contained the advertisement for
the Ozark Music Festival held in Sedalia, Missouri from July 19 to 21,
1974?

Jackflurry --

Isuue #164 with a cover date of July 4, 1974 (meaning that it was on
the newstands from June 21 to July 4).  Karen & Richard Carpenter, who
are brother and sister, are on the cover in a photograph done by Annie
Liebovitz:
Rolling Stone Covers (July 4, 1974)
http://www.rollingstone.com/photos/gallery/5392216/1974_rolling_stone_covers/photo/14/large/bobdylan

The ad is on page 55.

A check of five different issues for that year (including May 9, May
23, June 6 and June 20) shows that it was the only time that the
full-page ad for the Ozark Music Festival ran in Jann Wenner's
magazine.  Tickets were $15 in advance (with orders needing to be in
by July 1) and $20 at the gate.

You probably already know this but the headliners show how the state
of music changes.  Headliners were Bachman Turner Overdrive, REO
Speedwagon, The Eagles, the Ozark Mountain Daredevils and America.

Bottom of the bill: Bruce Springsteen and Bog Seger.

Google search strategy:
find a library with Rolling Stones archived back to 1974

Best regards,

Omnivorous-GA

My question was answered to my full satisfaction.  Thank you so much.