Alternator light shining brightly

[P'bert]

This evening, the "battery" indicator light in our '85 300D came on brightly, while the two indicator lights to its right glowed at half brightness. I measured the voltage at the battery, with the engine running, as 12.3vdc, not at all good. With the engine shut down for a few minutes, the voltage rose to 12.6, which suggests the problem I' m having began very recently.

Alternator/regulator? Other thoughts?

Thanks,

Phil

'85 300D, of which I write here
'84 Euro 300D, trouble free today