[Jim Smith]

After analyzing of the conditions and circumstances, discussions with several fine mechanics and Skymaster specialists including Owen Bell and Don Neiser, we've all come to the conclusion the only way this could've occurred is through one of the fuel valves. How, we're not exactly sure. Possible trash, weak spring(s), bad o-ring(s), or internal corrosion. My valves look beautiful on the outside but are original and have never been rebuilt so who knows what the insides look like. There is the possibility that one valve was slightly out of detent but it would've been only ever so slight. My future course of action and recommendation to other owners is, during your preflight make sure you hear each valve click as you put it into position, if it had been previously moved out of position from the mains. The cables from the valve selector handles to the valves do tend to get some slack and play. Adjust if necessary. Additionally, even though I'm not aware of any Cessna SID or an AD, it would be prudent to remove your valves and go through and rebuild them if they've never been rebuilt. At the very least inspect them internally. Being a 1969, mine are 44 years old and I'd have to imagine there may be some stuff going on in there that probably isn't good especially where there's any rubber concerned. You can maintain your aircraft in tip-top condition and one thing like this, that isn't required to be rebuilt unless it's broken, can take you down. There have been too many Skymaster crashes attributed to fuel exhaustion and who knows if this could be part of the reason why. Also, if you have a fuel flow/totalizer system, these add a great level of confidence with fuel usage, but continue monitoring for uneven burning from your good old fuel gauges. Thanks to everyone for your input.