Deformable metallic or plastic connectors of the type having an elongated bridge carrying two mirror symmetrical rows of substantially C-shaped prongs can be assembled with successive stacks of overlapping sheets, wherein the spine of each stack is provided with a row of straight or arcuate holes, by causing the connector at an inserting station to move along an arcuate path extending transversely of the bridge and having a curvature corresponding to or approximating that of the prongs in one of the rows or that of the arcuate holes. This results in the introduction of one of the rows of prongs into the holes. The prongs of at least one of the two rows are thereupon deformed so that the tips of the prongs of one row abut or at least slightly overlap the tips of the prongs of the other row; this prevents accidental separation of the connector from the stack.