'65 and '66 were factory equipped with shrouds. Non-AC 6-cylinder models did not get shrouds, but instead had rubber sealing strips added to the radiator support to increase airflow through the radiator. They were some of the first to get them, as previous years (except heavy-duty applications) did not have them. As shrouds were a 'new thing' to many mechanics and owners, they were often left off when the first V-belt change, hose change, or other work was done, as they were 'in the way'. Why not - earlier cars didn't have them, right? So, many cars lost them in the first few months, and it didn't matter so much as everything was all fresh, new, and still worked well with no bugs in the rad, corrosion or scale to interfere. That was almost 50 years ago, and things are not so fresh now.
If you look at concourse restorations, you'll see the shrouds that came on them originally. Finally, whether you care about that or not, fan shrouds are enormously helpful in increasing cooling, and should be used whenever possible.
:tup:
David
Original steel shroud - V8 only - part number C3DZ-8146-D:
Plastic version also used on I6s with AC - part number
C3DZ-8146-F: