The 40 cent documentary stamps were usually used by insurance companies on pricey policies or by brokers to make up fractions on taxes of large stock sales. I don't yet know who D.M. Co. are, but I like the wavy style of the box around this straightline cancel.
There was a company during the time of the issue of this stamp called the Dental Manufacturing Company that was one of the most significant players in the world in the dental implement business. But other than the box looking a bit tooth like I have nothing else that would point to this company having cancelled this stamp.