Hi bobh,
if we're talking about Duc E llier alternators, the diodes are always inside the casing.
That plastic cover clipped onto the back of the alternator has a couple of holes in it because some versions of these alternators had exposed pins which provided unrectified voltage to the 'magic box' that controlled a (lack of) charge warning light.
However, the 'sine wave signs' are present on all of the plastic covers.
If there aren't a couple of brass pins poking out of the alternator, similar to those on an ignition coil, you can breath easy...
ken