With C60X-03 dated January 1944 and C60X-04 dated January 1945 it seems that any change to Carter carburetors for the C60X would have happened (or possibly only been planned to happen) between those dates.
My list for the Fox (CAC-02, dated February 1943) also makes no mention of a Carter option.
In a listing of Approved Vehicle Parts Lists for Canadian Army Vehicles dated September 1949, CAC-02 and C60X-04 are the last published versions of the parts lists mentioned for the Fox and C60X respectively.
Based on these lists I would guess that the Carter could have been used in late C60X but wasn't likely used on the Fox. This doesn't mean that the Carter might not work well in a Fox since they both used the 270 engine, but there doesn't seem to be a parts listing of production of Fox with Carter.
It is quoted that there were 2710 C60X produced with about 1500 going to Russia. CDLV407 dated Dec 41 was for 500 vehicles (I believe this was the start of series production)
http://www.mapleleafup.net/forums/showthread.php?t=7491. C60X produced in Aug 42 seem to have been CDLV 587 and Dec 42 to Jun 43 CDLV1516. I don't know how many were in the last 2 contracts mentioned, which other contracts existed for the C60X or the dates and numbers involved.