yes.
now clear enough for me also... tks to both of you...
I should have read Palmer's books with more attention...

(ok, quite difficult to read and remember every single paragraph and caption...

)
well... now I learnt that buddy seat was no intended for a 42WLA, even if civilian later bikes had them, together with luggage carrier, as accessories... up to dealers to find a way to assemble them together (hook assy was the main problem).
no WLC equipped with buddy seat as well (only tandem seats)... and images on WLC manual are from 41WLA touched up (tks to Bruce's book...).
back to me... I'll have to bring my WLA to DOT check with a solo seat but with, at least, the buddy seat supports assembled on my bike (that's 'cause of documents I was released from italian ASI, and that's why I was trying to have the hook placed into position)... I will assemble hook and yoke/springs assy as we finally considered.
at the end, anyway, and as soon as I'll have both clamps sizes available, I will try to place the buddy seat on...
the only part I missed, right now, is the bumper... I found it original, but 130euro for it (in Japan) is definetely too much...
ciao
maurizio