The Z71 package first appeared in 90 or 91 ( I used to know which year for sure).
The package consisted of skid plates under the the front diff and T-case, front sway bar that was 2 mm thicker, Bilstein shocks........and a decal. Every Z71 I've seen had the 3.73 gears (but 3.73's were practically standard for GM 4wd's sold in the Rockies). The locking diff was a common option with the Z71's, but neither of mine came with it.
As mentioned, the aluminum/plastic panel between the bottom of the radiator and cross member is for air flow and does have the added benefit of preventing brush or weeds from hitting the fan.
And no, dealers didn't surreptitiously apply decals to trucks without the package.........