Definitions and time frames for the three phases can vary and can be confusing, at least to me. In the book, A Study of Navajo Concha Belts, by Donald Richards, Kirk Smith belts are all listed as Phase 3 probably due to date of creation, but also the belts’ style with closed-center conchos and butterfly spacers with stones.
This thread might be helpful: Navajo Jewelry: 1st, 2nd, and 3rd Phase Time Frames?