Ford never used the GE part numbers for their bulbs. A person can still locate the GE miniature bulb catalog and go by specs of what you have to fit. It could be a #44 or 47 but a person needs to know the type of attachment and the diameter of the bulb or at least the diameter of where it has to fit. The GE 51 was used a lot so check it out too.
There are bayonet types, screw in types, pin types, and various others. Most of the 6-volt ones are older numbers. GE had a number rating for the diameter of the bulb and that one looks like a G3.5 or T3.25. If it is smaller then in could be a T2.75.