Mills was the most prolific designer and manufacturer of early
aluminum head golf clubs. He created The Standard Golf Company, a
division of The Atlas Works, to produce his clubs which are commonly
referred to today as "Mills" clubs.
This Mills MSD 3 1/2 Model was produced in the early 1900s as special spoon. Among other things, Mills sought to eliminate the use of irons, so the MSD 3 1/2 was actually to be used in the place of an iron. The Standard Golf Company's 1909 catalog recommended this club "for players who prefer a club more laid back."
As is typical with Mills clubs, the lie (standard) and head weight is stamped on the sole. The 38 1/4" original shaft also has its original leather-wrapped grip.
For more on Mills clubs see TCA2 v1 p338-341.