Yes, I have been trying to find out some information about this chambering also. Everything that I can find suggests that there were only three chamberings. The .308, .284, and .243. I did find one article that suggests there may have been a very few number of 100's chambered in .358 but the author has never personally seen one in original condition.

I strongly suspect that this one is a rebarrel and refinish job.

Would definetly be a cool little gun though.