This site may help and / or give some clues on No6 battery refit.
Leclanche' is the technical name for (credit to the inventor Georges Leclanche' in 1866) the dry cell. These 1.5 V cells were a safer alternative to the wet cell (lead-acid) which were co...
You 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.