concur with your statement:
... 'original Lucas' rated the RM21 for 10.5A @ 5,000 rpm and, as I say, the RM23 for 14.5A @ 5,000 rpm; 75% is 7.875 Amps and 10.875 Amps respectively, an extra 3A at low rpm is a bit more than "differ very slightly" on these old heaps.
You are however, placing much faith in the advertised figures.
The quote says "rated".
Triumph, BSA and Norton
had enough faith to repeat the ratings (or the corresponding 120W and 180W) in their contemporary advertising (somewhere in the electric-start Commando
advertising is "15A @ 6,000 rpm").
The standard Triumph workshop Manual
test for an RM21 is minimum 9A AC @ 3,000 rpm with a 1-Ohm resistor between the stator wires; those more familiar with BSA and Norton
workshop manuals can confirm what they used.
Promotional ads from Lucas in ~1980 did suggest that the 3-phase system would solve all my issues (which it did).
High-output 3-phase was advertised from launch in summer '78 with the previously-mentioned 75% and 85% @ 2,400 rpm, presumably to indicate the superiority of 3-phase. You placed faith in the advertised figures and have positive first-hand experience.
If the system voltage is at least 13V at 3000 rpm with the headlight on,
"13V" based on ...?