As far as Rover V8 engines are concerned 'Pre-Serpentine' refers to the 3.5 / 3.9 age of engine design. These engines were originally produced with a multiple ancillary drive belt setup, one belt for alternator, another for power steering, water pump etc. The drive pulleys normally have a single groove. These engines use a 'long nosed' camshaft that is fitted with a drive gear to drive the distributor and have an oil pump which is driven by the bottom of the distributor shaft.
For these reasons, it is important to distinguish between these engines and the later 'serpentine' engines so that the correct parts are supplied or fitted.
The 'intermediate' version of the front timing cover is an oddity that was used during the transition between 3.9 and 4.0 Litre engines. These engines were fitted with a single multi grooved drive belt that 'snakes' around all of the ancillaries on the front of the engine, but were also equipped with a distributor. The distributor is still driven by a helical gear on the front of the camshaft, but the oil pump is crank driven and not connected to the distributor. These engines use a long nosed camshaft but were also usually fitted with a camshaft retaining plate.
The 'Serpentine' front cover does not have a distributor hole at all. The camshaft is short nosed with a keyway cut into it to receive the timing gear. This version has a half moon camshaft retaining plate fitted.