Overdrive relay question

[engine103]

1953 Mercury 6 volt +. I'm replacing all the old cloth wiring for the overdrive and while I'm at it I'm thinking of replacing the relay. I have the smaller relay that is on the firewall but also have a larger one that I've had for many years and can't remember if the larger one is 6 or 12 Volts. Does it make a difference if it is 12 volts? Can I use it in my 6 volt system? Thanks, Jim

relay 1.jpg

relay 2.jpg