An adjuvant composition comprising a C1-C2carboxylic acid ester of a polyhydroxy compound containing from 2 to 20 hydroxyl groups and an N-heterocyclic compound having a pKa, value of from 6 to 8 is storage-stable and suitable as an acid donor in the dyeing of polyamides. The panel has rows of spaced block support elements in the form of stamped, inclined, V-shaped prongs that extend outwardly from the panel such that when the gap between the blocks is filled with mortar, the mortar flows behind the prongs and thereby locks the building blocks on the panel. A valve door assembly including a flapper door is secured in the housing, and includes a seal between the flapper door and the housing.