I had a 6v starter on a 12v car and it did work until driving down to the 80th in France when having to ‘churn’ it due to the hot weather to get it started again after stopping for petrol, it gave up the ghost on the rally field as we were about to set off home….
As Bernie says a 6v will work on a 12v system, but heed his warning.
David Faulkner