a shroud need only enclose the fan blades, not the entire rad surface, and GM in fact made all shrouds that way early on
There are 2 different terms for that.
It's called, in engineering terms, a "mistake". In accountant-speak, it's called "reduced cost".
It happens when they decide that making a single duct (i.e., a device to enhance the efficiency of a fan, rather than a shroud, which by definition restricts and focuses air-flow) to save money, and apply it to as many radiators and chassis as possible.
It works fine if the engine:
1. develops no power
2. can use a fan far too large
3. gas is 20¢ a gallon
4. has a grill opening the size of a barn
They sort of stopped making these about 40 years ago.
A full shroud (as currently found on everything) needs a smaller radiator, smaller fan, and smaller grill.
