NavList:

Geoffrey Kolbe wrote: The problem is the cast alloy cap to the battery container, which takes the bulb. In my experience, the inside diameter of this needs to be enlarged (bored out with a lathe) to enable modern batteries to fit

I forgot the cap fitted inside the cylinder.  However, I’ve just checked. My widest Duracell C battery at 0.999” diameter is still a loose fit inside the cap.  I wonder if some kind person bored mine out in the past.  However, the published standard diameter of C batteries is 1.03”, whereas the inside of my cap varies from 1.018” to  1.028”, so maybe it’s worth trying a few different makes of battery. DaveP