Do you mean 1955-2nd series or 1956 (there was no second series 1956).
If it is 1956, it is most likely 12v. It came from the factory 12v. It is doubtful that someone would have changed it to 6v.
Does the starter and/or a generator have a metal tag on it? Is it red (12v) or black (6v). This is not "proof" because a 6v starter might have been used on a 12v truck (but, not visa versa).
Check a headlight (you might have to take it off to see if the voltage is on the back).
