From what I understand and have read is that the very early halftracks that came off the assembly line had a fixed idler with no spring. I'm guessing that this led to some track problems. the single spring ideler set up is a field modification that led to the dual spring set up that was factory installed. As for the two different ideler arms. I've also seen halftracks with different ones installed. I don't think there is a difference, just my opinion. Hope this helps.
Steve A.
