I just spent some time sifting through the Amfleet 1 roster in the book "Amtrak By The Numbers," and my eyes went buggy before I could locate your specific numbers. What I can offer to you is that the 43000 and 44000 series numbers were 2nd, 3rd, and some in the 4th renumbering Amtrak numbers. The original numbers were 20000 series numbers. Subsequent renumbering includes numbers from 20000, 43000, 44000, 80000 numbers.
It is safe to say that the MTH numbers you have could belong to phase 2, 3, maybe IV. There are examples of the first 20000 numbers lasting thru to phase 3 paint. Phase 3 paint is the 3 equal width red white blue stripes.
At some point when I have more time and fresh eyes, I will try to find your numbers. Although the first Amfleet numbers were in order, the renumberings are all jumbled in no order that I can yet understand. I will repost in the future if no one else sorts it out first. Here is a pic of one of the many pages of Amfleet numbers in the book to give you an idea.