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At this point, the flap was only blown by prop wash, I believe!
I presume the purpose of the rig was to confirm actual pitching moments, pre-first flight. I'll try to find out from somebody who knows later this week.
In the 1960s (I think) DeHC were interested in STOL. I think this was a non-flying testbed to examine deflected slipstream effects. Mounted on a frame to get the aircraft out of ground effects and with that grid painted on the fin it looks as if they were measuring downwash effects over the tailplane.
Here is the flying version.
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