At the very least, there are many sources that study the reflectance spectra of Munsell chips. One that is in the public domain is at . The server was awfully slow when I tried to access the links with the ASCII data files; I had better luck with ; click the squares for reflectance data. This website is cited in, for instance,

Fernando Ayala, José F. Echávarri, Pilar Renet, and Angel I. Negueruela, "Use of three tristimulus values from surface reflectance spectra to calculate the principal components for reconstructing these spectra by using only three eigenvectors," J. Opt. Soc. Am. A 23, 2020-2026 (2006) (abstract: ; the article cites the website above as Ref. 24).

One of the classic sources—also cited in the paper above—seems to be

Kenneth L. Kelly, Kasson S. Gibson, and Dorothy Nickerson, "Tristimulus specifications of the Munsell book of color from spectrophotometric measurements," J. Opt. Soc. Am. 33, 355-376 (1943) (abstract: ).

The Kuehni book I mentioned above lists a bunch of other sources. You can browse it ("search inside"), if you have an amazon.com account (used at least once to purchase something from amazon.com itself), here: . 

Incidentally, I now realize that these sources may give enough information to settle the very question I posed. But to really settle it, I would first need the answer to the following: was I wrong in the identification of the Munsell's color in my initial posting? That is,

What is the identity—in the Munsell system—of the Munsell color to which the Wikipedia article is referring? Is it in fact 5RP 5/12?

Having realized my probable mistake above, now I don't understand something else. Look at this statement in the article:
…professional artists, following Munsell color system… regard purple as being synonymous with the red-violet color shown at right, in order to clearly distinguish purple from violet…
In what sense are artists supposed to be "following Munsell" here? Obviously not in nomenclature. Is it the case that the artists do use Munsell's colors in their work, but change the names (e.g. Munsell's purple → artist's violet, and Munsell's red-purple → artist's purple)? I'd rewrite this if I understood what is attempted to be said here.