I absolutely can see someone adding a power booster. It’s not uncommon at all. It’s amazing all the little things you’ll find that people have added to a 50 year old car, and if they added it 30 years ago it will look original.
If anything, 50 years ago or so I would expect someone would have been removing the booster, not adding it. The OP's Dad doesn't strike me as someone that did more than drive it, so it would have had to have been prior to that.
At the same time, no idea why it wouldn't have been on the fender tag. Being 10+ years old when the OP's dad first got it (best guess), anything could have happened. Maybe PB was missed when the car was ordered and dealer added it?
In the end, there is photo documentation and a reasonable assumption that it was on the car when the OP's dad owned it. So I would leave it on and enjoy the car the way it is.
