There was a change from the spire nuts to the friction bushes sometime in the mid-1960s. I admit that I prefer the friction bushes. As Darrell notes, the spire nuts can leave the part free to vibrate, although that can be slowed or stopped with a dab of black silicone or similar goo on the backside. The bushes tend to hold letters and badges very snugly, with less damage to paintwork around the hole.
The only question I have is one that I've had in the back of my mind to try to answer for many years now is: Are the holes in the panels ever so slightly larger where friction bushes were used? Anyone know? (Someday I'll try to measure each type and see....)