Almost all Mason merganser drake decoys are common or American, figuring one model would substitute for both commmon and redbreasted. The hens were painted solid brown above their waterline. In fact, the species are differentiated by a white chest on the common hen. This example, and perhaps 5-6 others I have seen, has a more accurate white throat consistent with a common merganser hen.
See our "Mason Decoys" book, page 73, for a similar example.