The earliest engines had no fan clutch at all. The fan was driven directly from one of the belts. If the engine speed changed, the fan speed also changed. This method is very effective, but does constantly consumes engine power. Moving all that air around isn't easy.

In an effort to make more power available for turning the rear wheels of the car, the factory incorporated a clutch into the fan drive system. With a fan clutch, the fan is driven only when the coolant temperature is higher than a predetermined level. When the coolant temperature is within a "normal" range, the clutch is disengaged and the fan blade floats, reducing the load on the engine. 

The fan clutch is an electro-magnetic unit. This being the case, the fan clutch is either fully engaged or fully disengaged. There is no intermediate level of drive to the fan blade. The clutch is controlled by a thermo-switch mounted in the top of the water manifold.

The direct drive system was installed on:

Engine type 189.984-10-xxxxxx up to number 000131.

Engine type 189.984-12-xxxxxx up to number 003651.

Engine type 189.985-10-xxxxxx up to number 000166.

Engine type 189.985-12-xxxxxx up to number 000665.

All 189.984 and 189.985 engines after these points, and all 189.986 and 189.987 engines had the electro-magnetic fan clutch installed.

The exception to the rule occurs when the car was equipped with either factory installed, or proper dealer installed, air conditioning. In this case, the engine was equipped with a hydraulic fan clutch. This applies regardless of the engine type.

The clutch is driven by pressurized engine oil being directed through the clutch by a temperature controlled valve in the cooling system. The hotter the coolant, the more the valve opens and more oil flows through the fan clutch.

There are two types of the temperature controlled valve. The early type is mounted on the left side of the cylinder head, about two thirds back the length of the head. The later style valve is  mounted on top of the chain tensioner for the chain that drives the injection pump. It is located at the front of the engine, to the left of the cylinder head.

I have not yet found anything to indicate exactly when the changeover occurred. The one piece of information I can offer is that cars up through 1964 have the early style valve, and 1965 cars have the later style valve. This implies that all 189.984 and 189.985 engines and early 189.986 and 189.987 had the early style valve.