What I understand is as follows:
Duane Waterman is a luthier of long-standing, originally building in or around Kansas City Missouri , later Colorado Springs. On his own I believe he built mostly classical guitars. He then spent several years with Tacoma Guitars, doing design & development of their early models, such as the ThunderChief and the EM-9, as well as the Papoose and the Chief . He left Tacoma to head up LMI, a tonewood supply house. He is now in Tucson making Sonora guitars.
The instruments I've read about that were labeled D.M. Waterman were from his earlier period. But I don't know when or if he stopped using that label.
Elsewhere on the 'net there was a guitar with label a small label inside that says "D.M. Waterman, Fine Guitars, Colorado Springs, Colorado" that was identified by a poster as having been built by Duane Waterman.
And a dulcimer labeled [only, I think] "D M Waterman". In this case, the purchaser actually spoke with Duane Waterman about it and confirmed that Duane built it when he lived in Missouri.
Yours, I can't say. But it strikes me that how many luthiers named D M Waterman can there be?