It had to do with the pattern on the face of the bulb lens. The pattern on the face will focus the light as it passes through the glass. I'm hoping someone finds a good picture of the two different styles side-by-side and post it. The narrower/tighter pattern that they developed later on would focus the light a little farther out and you might see them advertised as +40 bulbs because they would supposedly give you 40 extra feet of driving light...
here are the 2 lamps with wide and narrow flutes. Top photo is narrow. The bottom one is one from my Superbird. For interest, the date code stamped on the back is 6-9 (June 1969)
