The distinction is vanishingly small. Every new memory technology (paper tape, punch card, drum, core, rom, prom, eeprom, magnetic disk, NFS, NOR flash, NAND flash, stone tablets, etc, etc) would then generate a "first computer to store on X". X is just a storage technology. Heck - computers from the 70 still stored programs on paper punch card and only loaded a few opcodes into RAM. If you are aware of what cache hiearchy is, then this distinction becomes even fainter - the Z3 might have some instructions cached as relay state - electrical storage.