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This site may help and / or give some clues on No6 battery refit.https://sites.google.com/site/johnengsdeadtechrescue/photos/john-s-ideas-to-restore-dead-technology/vintage-no-6-dry-cell-comes-to-life-againYou could use single D cell battery holders wiring 2 in parallel or you can solder directly to the terminals but a very hot Iron is needed to make the join quickly, too slow and the cell will be damaged.