As with all things in life I may be completely wrong on this, but I think the 855 change was only for fitting the larger center mounted fuel tank, and not a chassis size change. I know early 855 owners with the 72 inch MMM. That deck would not fit on the shorter 755 frame. I have seen an early JD studio photo of a model that is 755 size but with an 855 label that could have been an early design thought. As someone with an interest in the x55 tractor history, I would be glad to learn of further sources of information on this.