any given year the factory installed them as required by the laws in the area the truck was to be sold, it wasn't a GM decision or an option, cren cites the correct locations for the ID or marker lights, clearance lights are as close to the corners as you can get ..... aside from having duals those "smaller" trucks no doubt originally had flat/stake decks 6.5' wide with clearance and ID lights on the back as well, for the intended purpose .... which doesn't rule out the possibility that many of the trucks you've seen had lights added by owners, I've seen 1/2T trucks with half a dozen lights evenly spaced across the front of the cab roof
Bill
