Firstly, let me echo Paul's thanks for everyone's info. I was rushing with my last post....and am still doing so, but there is small gap tonight so, at the risk of making the same or similar mistakes, let me see if I can expand a few things (some of which may have been superseded by later posts, but perhaps worthy of clarification anyway) ...
Just so everyone understands the relationships:
1. Paul (aka Hobby) is a good friend of mine and I am helping him preparing this Ducati 250 Diana Mark III (note that DCT-Bob!

As some further info:
The engine has been rebuilt by another local good friend of both of ours who used to spanner in his own Ducati shop. HE also owned Bruno Spaggiari's 1972 Imola bike (yes, that very one) back in the late seventies and early eighties.
In answer to DCT-Bob's earlier question, the kick start shaft was missing too, but I personally am not aware of what was in it's place. Paul has offered that the previous owner removed the k/s and if he did, then he removed the shaft (and even the disengagement ramp) too.
I do understand the Mark III/Mark 3 nuances and I'll try to train myself better, but bear in mind that this distinction (whilst valid) seems to me to be predominately a US one and the rest of the world. (where I am) had a more "relaxed" nomenclature for the variances.
DCT Bob Said -
__ As for ALSO keeping your battery from discharging,, all you'd need to add is a couple of power-diodes, so as to separate & steer the neg & pos halves of the AC.power between the ign.system & the battery. _ That way, a significant portion of the power that's otherwise WASTED by the ign.system, can instead be utilized to keep the battery voltage-level well charged-up ! _ A real no-brainer, actually !
Yep. And I totally agree. The issue right now for Paul is that we have to get this Duke sorted this weekend in order to get it registered locally and then crated up and sent to Italy for the 2014 Giro. 5 weeks shipping, God knows how long in Italian customs. The upshot of that is that we are forced into an immeidate expediancey, but may be able to deal with nicities later. In summary - we'll be implementing an electrical system tomorrow that will get us through West Australian registration next week. So It will be minimialist.
" This is the bike that ran the ignition system through the tail light with a small switch in case the tail light blew. "
____ NO, it is NOT ! -(Providing that your alt/mag.stator is actually the older/two-wire model.) _ As THAT Mark-III model just came with THREE wire-leads ! _ As IT needed a way to also power an added brake-light circuit.
I agree, Bob. Not sure where Paul got that from, and his two-wire does not have the switch, nor any native capacity for a brake light.
" (and I'm an IT guy and know how to use search engines) "
____ Then you seem to be someone good to ask about why many search-engines will (stupidly!) round-up EXCESSIVE-results which also individually includes the finding-result for each & EVERY single solitary word entered within their search-box, all independently,, instead of JUST results which includes ONLY that of ALL the words combined & entered !?
I'll come back to you on that later, but don't expect a lot of good news!!!
" I have trolled the internet on quite a few occasions now, and can find no such wiring diagram anywhere. "
____ Before considering all that work, you should've asked here FIRST !
Then we ALL may've sooner been made aware of the FABULOUS-example of an early Mark-III (which YOU already knew-of).
Or have we already indeed seen this particular example posted before ? _ As I seem to recall wondering whether-if such an installed front-fender is an actual stock early Mark-III part or not,, (cuz it could possibly be, [I suppose],, that it too, [along-with the headlamp, etc.], was borrowed-over from the preexisting Motocross model-line).
I'm pretty much certain that I've posted pics before. I actually did try to find them and just point at the earlier post, with no success. (This should give you some indication of the disappointment coming regarding the previous point!)
" I also understand that this model was only sold in the USA. "
____ Of that I'm not really sure of either,, but it's also as well been my-own understanding that the original bare-bones Mark-III version was only made just for Berliner, and-thusly imported only into the U.S.
That's my understanding as well.
" I have now figured out the wiring so this weekend it's testing time to see if all works. "
____ By "all", do you then also have a stock handlebar-switch to connect-up along-with the rest of the minimal stock wiring ?
The rest of us here only have that 450-scheme diagram you posted to consult,, so when you say you have figured-out the wiring, we're left to assume that you intend to power the taillight-filament,(LED) with the same power-source as the headlight will be powered by.
And if so, it would be of certain interest to learn of how well your tail-LED performs with the stock AC.power setup.
So could you please elaborate on this matter ?
Just to clarify a misunderstanding... the headlight, high beam and tail light will be powered by the stock "magneto" system. This will be wired through to a CEV three position (plus horn button) switch.
Only the horn and the brake light (which were never part of the original system) will be seperately powered by a 12V battery with earth circuits returned to the battery (not the frame). The brake light only is LED. The tail light and headlight circuit is stock off the "magneto".
Hopefully all will go well tomorrow!.
Kev