The envelopes were assembled by ITS employees based on the concentration camp (abbreviated as KL) from which the documents came. There are envelopes printed with KL Dachau, KL Flossenbürg (sometimes incorrectly spelled Fossenbürg), KL Buchenwald, KL Mittelbau (for Mittelbau-Dora), KL Stutthof, KL Niederhagen-Wewelsburg and the Amersfoort police transit camp (Polizeiliches Durchgangslager, PDL). If the material came from camps for which there was no pre-printed envelope, the information was added by hand.