More than 600 items stolen from a city museum are irreplaceable, an academic has said.Simon Potter, professor of modern history at the University of Bristol, said the items missing from Bristol Museums’ British Empire and Commonwealth collection are “unique” and of “huge value” to historians.Four men are believed to have gained entry to a building in the Cumberland Road area of Bristol, which houses the artefacts, between 1am and 2am on September 25.