I thought those were spare flashlight batteries...

Yeah, since all the interior lights were on one circuit breaker that would come in handy if you were in the dark.  Uh, luckily, the Skymaster I flew had an "ice detection" light inboard of the left strut - turn it on and if you see ice turn it off.  However, the loss of the interior lights could be overcome by turning on the ice light so you didn't have to hold the D-cell flashlight in your teeth.

 

Hopefully past the past,

 

Grayhawk