Right On Roger :-

The reason for how and why is the difference in material of diodes.
SILICON Diodes have a Voltage Drop of 0.7 VDC
GERMANIUM Diodes have a Voltage Drop of 0.3 VDC
If you want a safety margin with the use of these Germanium Diodes just put
each bank of diodes in PARALLEL.  This will DOUBLE the
current handling capacity and if a single diode in the parallel pack blow
open it is protected by the other.

The numbers you have for Charge requirements are very close... Probably a
typo...
13.8 Min to 14.2 VDC Max.  YES you will surely charge at 13.9 VDC :-)
With some charging systems you may see as high as 15.2 VDC but that will
drop as the battery is charged.
BUT!  Correcting this problem with a low voltage drop diode still makes me
wonder ... WHY is the output so low that you have to play games to SQUEEZE
out the extra 0.3 VDC to put the SYSTEM into a CHARGE state?
Are there ANY adjustments to the regulator to UP the voltage?
Basic design says the alternator output should should be high enough so
regulation is performed not at the edge of failure.  The output of many
alternators is in the range of 20 to 30 VDC BEFORE REGULATION.
I would definitely question the alternator manufacture why things are so
close to the edge of failure.
In my experience of charging systems in GA craft the BIGGEST problem has
always been CONNECTIONS - GROUNDS in particular.  You can not visually
inspect these connection.  They MUST be dismantled, cleaned and re-assembled
with new star washers.  Star washers - NOT - Split ring washers.

So - GREAT FIND Roger.

Barry