I'm delighted to report that after a few months of research into the submission status of HOUSE OF DARK SHADOWS, in regards to its submitted and released length as approved by the Japanese censor bureau.
Contacts at the PR firm that handled HODS in Japan when it was released in March 1971, have confirmed that the Japanese version WAS INDEED A LONGER VERSION THAN THE US one. It's length in Japan was at least 4 mins longer, running 101mins. The only prints that currently exist run just under 97 mins.
There are conflicting reports on the contents of the longer version, which have been reported over the years, but the most likely case is that this version would include the 'David hanging' sequence, which encompasses several new scenes added to the beginning of the film, which offers a new introduction scene to virtually all the main characters in the cast. Unfortunately, Japanese sources could not confirm the scenes exclusive to the Japanese release.
I've started the ball rolling and have begun further research into the Japanese release, which will include inquiries into Japanese studio film element holdings and discussions with underground Asian film collectors.