I think you might be thinking of the mass retro fit performed on the exported Mk.2 and Mk.4, where the slatted, narrow chord wing was replaced by the hard/fenced 6-3 wing before delivery to other air forces. I’m pretty sure all Mk.5s produced retained the hard edge wing throughout service life, even after being returned to Canada. Initial Mk.6 aircraft also had the hard wing but were retro fitted with the slatted version of the 6-3 wing after it became the production standard at Canadair.
As far as I know the only “Mk.5” with slats was Hawk One, and she was/is a Franken-Sabre: Mk.5 aircraft with Mk.6 engine (Orenda 14 vs. 10) and with American F-40 (slatted, tip extensions, two pylons) wing.
BTW, it would be interesting to hear from a Canadair employee from back in the day about the model numbering convention, I’ve seen both styles of numeral in print. I choose to use Arabic vs. Roman numerals (Mk.6 vs Mk.VI) because I’m lazy :)