Hi Rich, did you ever find the solution to this curiosity? I just received the 2021 Guide Book and the same problem is still there, no listing for the 6-51000 or the 6-51001 (#44 Tinplate Freight Set made in 1989). Looked under the tinplate section in the book as well but I believe that these two sets were not made in association with MTH under the "Lionel Corporation" name so they really should be in the regular listing section.
I will keep digging around and post an update if I find something.
Regards,
Walter