For a while North American Aviation pushed the idea of using the XB70 as a mothership for the X15.
There is a book called "Valkyrie" ISBN 978-1-58007-130-7 with an intriguing plan view drawing by somebody called Tony Landis of a XB70 mothership
carrying a modified (delta winged) X-15 on it's back. Also on the same page, there is a mock up image of a space plane (again resembling a delta X15)
launching of the back of an XB70, this composite is credited to Boeing hystorical archives.
Now I know this is all a re-telling of the brilliant buzzard stories but I can't help feel there is a story here and McDonnell Douglas certainly have
heritage in the space plane business
Isinglass was more likely an air launched, rocket boost phase, glider of some description. (by this read B52 style mothership / big bloody rocket,
booster not an XB70 derivitave mothership as that is a different story!
It seems increasingly likely that this thing was real, who knows maybe we'll hear more about it sometime soon....