There are a small handful of articles which profile Takayuki Tochio, the Panasonic image engineer who, according to many accounts, was the person singlehandedly responsible for the creation, development, and algorithmic implementation of the l.monochrome.d mode. Tochio has said in interviews that his initial process involved buying an old Pentax film camera, and shooting a lot of Tri-X 400, which he then developed and printed on various papers. In another interview, he said: "In order to understand the tone of black-and-white film, I bought film, took photos and printed it, and analyzed the printed matter and film from both sides. I have made various discoveries. In particular, I have learned from my experience that the impression and performance of monochrome depends largely on the way it is printed on photographic paper, rather than film development. Therefore, I decided not only to refer to the film itself, but also various photo books to design the image quality." He also added, "...compared to L. Monochrome, L. Monochrome D emphasizes the tones of highlights and shadows, while adjusting the overall balance of halftones to create a sense of three-dimensionality."
As someone who, in my analog film years, literally shot dozens and probably hundreds of rolls of Tri-X 400, which I developed and printed on a variety of papers, what Mr. Tochio said, resonates with me. It also partially explains my own fascination with this particular in-camera picture mode.