Vintage Heinz Ostergaard dress

Great condition! I cannot find any flaws with missing beads on this dress. Beautiful black and white knee length dress. There is no tag to show the type of material but the outside apps to be a heavy linen cotton type material. Dress is fully lined on the inside. This designer was popular in the 50s and 60s. Tag shows size 42. I tried the dress myself and I’m a medium 8/10 bra size 36C. The dress zips on the side but I was unable to zip. I would guess the size is more of a small 4/6.