I eventually got this response from Steve James...

"I actually visited Sam at home in 1975 (just before his tragic death) for a guitar lesson. I can still play a couple/few of his motifs, and have recorded my adaptions of a few of his tunes. Also tanscribed arrangements of Buckdancer's Choice and Railroad Blues in my Blues/Roots Fingerstyle guitar book. "Victor Rag" (sic.) is Maybelle Carter's. Note that it's "Victor Rag" (as in the record label). Chet Atkins, and hence Doc Watson, got it from her." - Steve James

So, it seems Maybelle Carter wrote it, and named it "Victor Rag", in honor of her recording label, RCA Victor.

Tony