I've flow over a couple thousand hours with PT-6s on 3 different types of aircraft. I haven't checked for a "prop retaining bolt" yet.Checking for the prop retaining bolts on preflight must be a huge pain in the a$$...
The text under the photo says engine test bed for P/W.
PT6A-68 is the T-6 engine.
That's one of many different models of the PT-6. The word I got regarding the PT-6 was that it orginally was an oil pipeline pump engine and was designed to run for days at a time. Someone would come along every so often and check the oil and top it off and then it would continue to run. Apparently someone was looking for a small lightweight engine to put on an airplane and thought the PT-6 (or whatever it was called) made sense. This is, in part, why it's installed backwards, since it originally was not meant to go on an aircraft.
Planes are used as testbeds for a lot of things