In Russian souvenir sets - a leaf ((sometimes - small booklet) with the textual materials about the subject of the set.
In Spanish matchboxes - a printed card inserted on top of the matches in the tray to reduce their movement (self-ignition preventer). Were issued in sets.
Equvivalent Russian term ¬кладыш
The insert from the Russian Souvenir set
"The USSR inland water transport"
From collection of Stanislav Dmitriev