I mistyped. UK Flag = USA #6.
Need new glasses.
My source:
http://www.blaukatz.com/tables/basic-round-cells/
There was a much smaller cell with thread on the case designed to heat a wire hot enough to light the gas stove/fire/lamp
U14 109.5mm tall 51.2 dia, threaded top
An old #6 Flag
Two of these would run your ereader for a long time