Fuel in rectangular shaped tablets for burners "Esbit" — a copy. 10 tablets of the correct rectangular shape packed in cellophane are inside the cardboard box. The tablets are hygroscopic (absorb moisture), so they should be stored in a dry and tightly packed form.
Esbit is the brand name for urotropine tablets (hexamethylenetetramine). The substance itself was discovered in 1859 by Alexander Mikhailovich Butlerov, and the Swabian (German) inventor and businessman Erich Schumm came up with it as a dry fuel. He founded "Schumm" firm in 1933 and introduced dry fuel to the market under the name "Esbit" by 1936. The word Esbit is an abbreviation for the phrase "Erich Schumms Brennstoff in Tablettenform" (fuel in the form of tablets from Erich Schumm). Source: de.wikipedia.org
Esbit Model 9 burners were used in the Heer. Of course, the company also produced other models. Esbit still exists: esbit.de.