I don't know all the finishes that were available in the flat-backed, all mahogany D-25s made from 68~72, but I know some were labeled "BR" for "brown." Don't know if any were labeled "NT" for "natural top," or if the cherry finish was available...
The spruce-topped arched back versions made from ~74 on were originally available in "M" for "mahogany-stained" and "CH" for cherry stained (there may have been others). Later (in Westerly and Corona), there were natural spruce tops, bursts, and a number of other colors...
There were a limited number of all mahogany arched back D-25s made, most in 1973... some of these were also labeled with "M," causing some confusion with the stained, spruce topped version from '74 and later.
The best way to look for a mahogany-topped version is to:
(1) look at the date (determined by the SN). IF its 72 or earlier, all odds are flat-backed all mahogany. If its 74 or later, likely arched back with a spruce top. A date in 73 could be any of the three.
(2) look at good pics of the top... mahogany has a very prominent grain that shows thru the stain. The stained spruce tops show little grain thru the tint.
Good hunting!