Note
QST, Dec 1977, inside back cover.
QST, Feb 1978, inside back cover (FT-901DM)
QST, Jan 1980, p. 147.
QST, Apr 1981, p. 182.
The only difference I can find by reading ad copy is that the 901D has variable IF bandwidth from 100 Hz to 2.4 Hz, while the 901DM has variable IF bandwidth 300 Hz to 2.4 KHz. But...
The "D" and "DM" version are sold concurrently, with a $200 difference between them: QST, Jan 1979, p 192, for example.
So there has to be a larger difference in them.