Saint Joseph was on the inside face of a shutter, probably from a triptych. It was on the reverse of 'An Angel' and would have been on the left of the missing central image of the Virgin Mary, when the triptych was open. The opposite wing, also in the collection, shows 'Saint Jerome' with 'An Angel' on the reverse.
Joseph holds the staff which miraculously broke into blossom demonstrating that he (among the Virgin Mary's many suitors) was chosen to be her husband.