I believe the assumption is that at some point all/most Celt used on the Continental was a type of Q that over time shifted to or was replaced by P. Maybe initially on the continue P was more concentrated to the east of Q. This is a complex issue, but may be tied to where the Belgae came from? If this was inspired, at least in the north, by the Belgae also remains unclear. Given the nature of the langauge group, it had to have happened (that is the change itself), yet it's very unclear when and why this occurred. I do like a greatly reduced Quariti with a strong connection to the Chalons-sur-Marne area. However, this pulls them even further from the sea, unless they occupied a very large area.
By the way, heres another...
http://www.kelten.co.at/index_en.php?sub=inhalte_en
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