We discussed this truck in another thread once. As oldsub mentioned Chevrolet didn't offer assembly line Tandems until 1956, so the idea was this truck is a conversion from one of the jobbers used by Chevrolet back then. Such as Trucktor, Truckstell, Thornton, Truxmore,Fabco, or Grico. The 1954 Chevy COE, I had was a 5400 series with a Trucktor chaindrive Conversion. On my truck the original drive axle, from Chevrolet, & the trailing axle, were attached to their own set of frame rails, that were then slid over the original frame rails, all the way up to under the cab. The original frame was the length of a regular medium wheelbase 5400 series COE. With the frame rails from the conversion added it made the truck frame longer. That may be the case here. See my Tandem here,
http://rides.webshots.com/photo/1115990817052570246. My truck had a Trucktor ID Plate rivited to the very last frame crossmember.
