An air guide box of a paper production installation has a front broad side and a rear broad side, each having a width b. An extension device is attachable to a broad side of the air box guide and has a width of at least 0.25*b and a length of at least 10 mm. A surface thereof is defined with at least one recess having a maximum clear opening of at least 0.1 mm for the through passage of at least one high-pressure water jet or at least one laser beam.

An air guide box of a paper production installation has a front broad side and a rear broad side, each having a width b. An extension device is attachable to a broad side of the air box guide and has a width of at least 0.25*b and a length of at least 10 mm. A surface thereof is defined with at least one recess having a maximum clear opening of at least 0.1 mm for the through passage of at least one high-pressure water jet or at least one laser beam.