Maybe I'm missing something here, but is there any good reason not to call this a dark morph hemilasius?
Laurent - various reasons: too small (although difficult to interpret from Hodgson's measurements), partially feathered (rather than 3/4) tarsus, lack of greyer flight feathers on underwings.
Further details of this form were given by Portenko (1929, Uber den taxonomischen wert der formen der palaarktischen bussarde. Bull. Acad. Sci. URSS (7) 7: 623-652) which is available from http://www.mathnet.ru/php/archive.phtml?wshow=paper&jrnid=im&paperid=5377&option_lang=eng
Portenko treats it there as Buteo japonicus saturatus var. plumipes (note that saturatus is the form now known as refectus)