The South Kensington Exhibition proof would have been printed on thick white card, but should not have a Maltese Cross cancellation. It was not printed from a black plate. This one has a cancel, that would rule out exhibition reprints, unless the Maltese Cross was forged.
I suspect it is a cut-out from some print (album, card, brochure) that reproduced a cancelled example. The nicely rounded corners hint to this.