If you have an old Electric Tick Tock that uses Flag (#6, R40) you can use Zinc Air 90AH "R40" size cells. Only from factory, but a 7.5V 90AH Round Fencer has 5. Open it up and block the air holes on the spares with bungs till needed.
The largest single cell isn't the Flag any more
Weight: 1.2 ± 0.1 kg
Length: 82 ± 1 mm
Width: 82 ± 1 mm
Height: 182 ± 1 mm
A Flag is about 61 mm diameter and 163mm high including terminals.
Unsurprisingly the rectangular 9V 90AH are really 8.4 (Zinc Air) and very like 3 x 2 off "Type O" cells in a box. The "Type O" seems to be a squared off Flag / #6.
The first Zinc Air Batteries were made in 1932 by US Eveready (National Carbon Company / Union Carbide), now Energiser owned by Purina Pet foods. They were for "Farm Radio" batteries as well as other applications, though how exactly I don't know as they are 1.35 to 1.4V per cell. I suppose 3 is OK for 4V valves (Lead Acid 4V is actually approximately 4.5 down to 3.6) as it would be 4.2V down to 3.6V.
4V was more popular in USA and mainland Europe than 2V for battery valves.
It was the 1980s when they figured out how to make hearing aid versions.
In theory the dead Fencer batteries I have acquired to discover the insides could have the zinc paste refilled! Nice big carbon rods.
Actually one cell is quite a friendly way to run 1.4V battery valves. Retail the AR40 from a 5AR40 "7.5V" 90AH fencer work out at €6 each if you can seal the unused ones till needed. A 996 pack gives about 40AH if you wire the cells in parallel. But while the regular price is about €5, They are on sale as low as €2! But the Zinc Air are like a 1.35V to 1.4V PSU so the performance is far better to the end.