I was looking at an early z the other day and noticed that it didn't have the rubber strip underneath the bonnet that runs in front of the radiator. Was this a design difference between early and late cars?

As far as I know, this was one of the beginning "fixes" to compensate for the vapor lock problem the cars started to exhibit.

By closing off the path above the radiator, more air was forced through the radiator itself. Other mods included the fan shroud and the "splash pan" which goes on the bottom of the car.

2¢

Enrique