I thought it was common knowledge by now that everyone knew that Frid, Karlen, Scott, Hall and most of the cast was off in Tarrytown NY, filiming the HODS movie during the beginning of the PT storyline on sci fi right now.
Selby and Parker practically carried the show every day for 6 weeks. All though they did a fine job, the main cast's absence really hurt the show. Especially coming so soon after the awful Leviathan story.
I believe that Daughters of Darkeness was filmed sometime in the summer of 1970 after PT ended.