I suppose myquest to get to the bottom of the truth on these batteries is my ownership of a Pentax ES, which took a 544. But in 1971 when the ES was introduced, the 544 was the correct battery. It just seems odd that the 4 button cells inside the 544 case were undoubtedly mercury (or could I be wrong?) And I thought it odd that 4 X 1.35 volts = 5.4. And they called it a 544 battery.
I'm happy with Matt's assessment of the Hasselblad finder taking the silver 6V, but I still wonder what the original 544 was. Go to Radio Shack or anywhere 544's are sold, and they're all alkaline. I wonder if the original 544 was actually mercury.