Hi folks, I said that I stand to be corrected on the mounting of 653's and I am going to correct myself.
I was looking at a set of Marchal 653's on ebay, item 4587230317, and there are some good shots of the mounting bolts, and from these photos it looks like there would be no problem to mount the 653's on the grille brackets.
The photo shows that the mounting bolt is the same on the 653's as is on the 656.
So, that brings us back to the initial question, what is the difference between the two lights?
Dave