It's from the G1 version. They both had the same cab in alt mode, but Magnus's combined with his trailer as armor. They used that as the basis for all the repaints.
if you look at the G1 toy of Ultra Magnus he was a white Optimus Prime cab that combined with his trailer to form 'Ultra' Magnus, so the theory is that the white cab by himself is just Magnus.
The real world reason is that G1 Magnus was a white repaint of Prime with a diferent trailer. If your after a story reason there was never one given in the original G1 cartoon or comics as his cab robot mode was never shown. In Dreamwave's G1 his cab robot mode is seen and Prime calls Magnus "brother".
the short answer is before Transformers, there was Diaclone. the "Convoy" Diaclone toy was released as Optimus Prime. Convoy got an upgrade as "Powered Convoy" in Diaclone... http://1500bc.com/pretf/pictures/diacat_b_8_9.jpg and this toy became Ultra Magnus after a little bit of recoloring on Hasbro's part. (and look how helpful Devestator is being!) The original toy UM is based on was definitely related to the toy that Prime is based on. Beyond that its mostly fan speculation and some hints in comics as to how close and Magnus and Prime are to one another.
