A masonry block which includes two supporting parts and an insulative malleable member for interlockingly and tightly inserting between the supporting parts. The malleable member has a plurality of spaced grooves in at least one portion of each of its supporting part-engaging surfaces which is susceptible to binding. Its supporting part-engaging surfaces comprise otherwise thereover spaced convex surface portions for engaging the supporting part surfaces respectively to accommodate gradual enlargement of the supporting parts due to mold wear. For ease of picking up the block, the malleable member has in its upper surface a pair of spaced transverse hand holds, a centrally positioned longitudinal hand hold, and a pair of thumb holds positioned longitudinally outwardly of said transverse hand holds respectively.