Stanley Hershel Stahl did his PhD thesis at the University of Michigan, under the direction of Peter Hinman, and wrote a dissertation in 1974 with title "Classes of Primitive Recursive Ordinal Functions" (MR2624549). This is referenced, e.g., in this paper by the same author. The research content of the thesis, or at least part of it, has been published in the paper I just linked to as well as this one, but I would be very interested in reading the expository parts (especially as Stahl's exposition seems excellent).
Getting hold of this thesis might prove challenging, however. Obviously no copies are available online. The author does not seem to have pursued mathematics after his PhD, and Peter Hinman is, I imagine, retired. There is a copy at UMich library, but before I try requesting an interlibrary load or scan (I don't really know how these work), I thought maybe someone on MO happens to have a copy, or could offer advice as to whom I might contact.
Also, the MathSciNet entry (link above) mentions something called ProQuest, I don't even understand what they do, which is a bad sign, but maybe they centralize requests for copies of dissertations. Does someone have experience in this matter?