We report neutronics calculations for the most important natural uranium-fueled reactor types historically used for weapons plutonium production. These include an early design of the Hanford-type graphite-moderated and light-water-cooled reactor used in the United States; the Calder Hall-type graphite-moderated and gas-cooled reactor used in the United Kingdom; and the NRX-type heavy-water-moderated and light-water-cooled reactor, originally developed in Canada for civilian purposes but later used in India and Pakistan for military plutonium production. We show that while it is possible in principle to identify with a high level of confidence weapon-grade plutonium compositions produced in other types of reactors, e.g., light-water-cooled or fast neutron reactors, it is difficult to distinguish among plutonium compositions generated in dedicated production reactors fueled with natural uranium. This suggests that efforts to determine the origin of weapon-grade plutonium for a nuclear forensic analysis could well remain inconclusive without access to databases based on actual samples of the nuclear material.