Vince, With supersonic flow the stagnation temperture (resulting from slowing the air down from supersonic velocity to static relative to the test shape) would destroy a plastic or wood model. Most supersonic models are made from stainless steel in my experience.
Ramjet engines are limited to use at only mach 3 or a bit more because slowing the air down to subsonic speeds in the combustor raises the temperature far above the combustion temperature of any of the hydrocarbon fuels. In fact, the fuel actually cools the air by absorbing energy required to ionize the fuel, and any burning would occur well after the mixture leaves the engine exaust and cools down to the point where oxidation can take place.
Down at Eglin AFB there use to be a ballistic range where the model was fired down an instrumented range filled with still air. In their display case was a 40mm Plastic discarding sabot and inside was a small stainless model of an F-16. The range used a 40mm cannon to launch the test shapes down the range and the range was used for some of the very early testing of the F-16 shape in supersonic flight.
Having said this, you could still be right that it is part of an impeller. On the other hand, the threaded hole could have as easily been a set screw to hold the test shape on a rod which slides in the rear of the item for testing in a high speed tunnel.