On the photo in question , do look carefully at the cockpit position in the fuselage , i.e behind the wing l/e and also with a high and level sill line. Then note also the shoulder wing position , all significantly different from the Marskes illustrated in the subsequent photos. Marske has its cockpit (with a sloping sill line) , positioned in front of a rather more mid-set wing,
Then check out the early Backstrom flying plank drawing above. I think you might agree this could be a distinct hint of the mystery glider's origin.
A picture of a similar type from the Backstrom family is illustrated below:
