First K codes had transverse dual exhaust, which was used up until Oct/Nov '64, which was then switched to Arvinode until around March '65.
Yes, you're correct about the transverse system, which was first of three. It still does not preclude that the first production line, dual exhaust systems were installed in June of 1964 in "K" Mustangs only.
Arvenode components are shown in the picture (borrowed from NPD) below. I could not find a "non-proprietary" drawing to use.
Jim