I have an answer! With a little help from Mark Tomasko, author of "The Feel of Steel: The Art and History of Bank Note Engraving in the United States". Here's his response to my query:
"It is definitely not a print from a transfer roll. I have never heard of transfer rolls being used for printing, and remember, that would be letterpress, not intaglio. The way you can get a reverse print from an intaglio die is to print it offset, I.e., printing an intaglio image onto an intermediary piece such as a rubber blanket, which is then used to print a piece of paper."
So -- knowing that the final product would be offset printed, Fuchs & Lang commissioned an engraving to be made in reverse (actually a positive image on the engraved steel plate). My proof would have been printed directly from that plate using intaglio methods (hence the raised ink) as a quick way to see various color samples. I have seen other reversed proofs just like mine in numerous colors, which makes sense, since the labels would represent the ink color in the cannisters they sealed.
But the labels themselves were printed using that intermediate step, from the engraved plate onto probably a large rubber roller, then to paper, with the end result in readable form.
And there you have it! Btw, for those who have never seen a transfer roll, here's an example next to a flat plate that the engraving was transferred to.