322.png

: There are commercial digital bundles, such as Watercolor Spring Animal Wreaths on Etsy, that contain exactly 322 PNG images.

: Renewals for The Mystery at the Moss-Covered Mansion (Nancy Drew series). Other Potential Matches 322.png

: Renewals for The Bobbsey Twins at the Ice Carnival under the pseudonym Laura Lee Hope. : There are commercial digital bundles, such as

Specifically, in the 1968 July–December records, (indexed as page 322) covers renewal registrations for authors and titles such as: : There are commercial digital bundles

: Renewals for The Mystery of the Flying Express (Hardy Boys series).

While the Project Gutenberg file is the most likely "paper" or document reference, "322.png" also appears in other contexts: