How do I identify a 6v electrical system on an older British car?

Slightly off topic of Mopars I know, but I need your insight.

I have been asked to resurrect a 1954 Morgan.

The family (mom, son and daughter), have some background on the car, but not much help beyond that. I am not too afraid of the basic points ignition, etc, but it will DEFINATELY need a battery, as it has been sitting for 17 years. The battery is funky, but doesn't tell me if it is 6v or 12v, and I can't seem to find any info online...

When did most auto switch over to 12v?