6 oclock pisition is about the start pisition. Looks like there have been a rash of timming buttons falling off. Pull the Head.
Ya, I just fixed one for someone where the button had fallen off.
The truck ran fine untill all of a sudden it wouldn't, i did the wooden Dowell in the center hole test and I had a little up and down motion but don't be fooled, if the button is off it can lay there in the bottom of the cavity and cause some movement but no fuel pressure to injectors, excessive cranking can cause damage to the button, clip and fuel compression rod, (you should have a small mirror for this entire process).
Be careful that the fuel shutoff shaft assembly is reinstalled properly, the little tit at the end of that shaft must be in the slot of the fuel shut off plate (square plate inside the head). When reinstalling the shaft assembly leave the govener rod off, position the shaft lever at about 3:30, slide the shaft in and get it seated, install the screws, then install the govener rod in the leaver hole,make sure the govener rod will freely pop back and foward and rest in the foward position(the assembly is spring loaded) don't forget the govener rod retainer clip.
Be sure to read the other threads on this subject (and tm) as to proper head removal (such as lined up gear marks), you should install new o rings while you have the head off and a new button retainer clip may be required.
Disclaimer: this is off the top of my head and I may not have all nomenclature of parts correct, no tm diagrams in hand.
Hope this helps.