Definitely was an option for 55 1st on driver side only.
I have heard rumors of factory passenger side mounted spare tires over the years, but on close inspection every one I have ever seen on has been a home made dimple job on the fender.
Dual spares are popular on lowrider show trucks.
There are even several people in SoCal that offer a service to dimple the fenders for the spare tire.
They use a press & finished product is smooth just like the factory did.
The fact that your truck came from Calif. leads me to believe that the passenger side fender has been dimpled & is not a factory spare fender.
Even though the passenger side dimpled fender was not an option, that would not have stopped an aftermarket coach builder at the time from stamping out the fender for the spare tire.
And then there is the possibility that right hand drive trucks had a dimpled fender on the driver side.