I can't say why this never struck me as odd me before, but how did winged altars so thoroughly and suddenly disappear in favor of reredos around the counterreformation? Browsing Wikipedia Commons Cat:Triptychs I can't even fill the gap before Rubens' Antwerp altars, of which the Elevation of the Cross is no longer in situ; they don't seem intended as folding works. When we visited Ghent there was, besides the glass-encased Mystic Lamb, a Japanese-printed full-size replica, but it was placed in a side aisle, an exhibit rather than something possibly fulfilling a liturgical role. Maybe though it gets moved whenever the B minor Mass is celebrated ;-P