This is the Martha Washington half dollar. The Martha Washington obverse was designed by Edward R. Grove and the reverse is by Philip Fowler.
Examples are believed to have been struck as follows:
Nickel J2131 with an example offered in Heritage 10/14 sale.
Cupro-Nickel Clad J2132 with over a dozen known including the illustrated piece above courtesy of Bowers and Merena from their July 2005 sale.. An example was first offered in ANR's 12/03 sale and a second one was in Bowers & Merena's 7/05 sale and later reoffered in ANR's 1/06 sale. One is also known struck on a clad quarter planchet. A mint error, it was offered in Heritage 3/2003 and 8/2004 sales. To view this piece, click here. It is unknown if these were struck inside or outside the mint.
50% Silver, 50% Copper J2133
Silver Clad J2134 as used from 1965 to 1970 on the Kennedy half dollar. There are 6 examples, 3 each embedded in two blocks of blue lucite, along with J2101/P2081 and J2116/P2082 in the Smithsonian. To view one of these blocks, click here.
Pollock inadvertently lists these Smithsonian pieces as being struck in cupro-nickel clad.
Columbium (type 1) J2135
Columbium (type 2) J2136
Monel (nickel base alloy) J2138
Nickel-5% Silicon with Magnetic Core J2139
Stainless Steel (type 301-Type I) J2140
Stainless Steel (type 301-Type II) J2141
Stainless Steel (type 302) J2142
Copper Zinc Silver alloy J2143. In March 2016, NGC received an example for grading that is Cu 58.3%, Zn 34.3% and Ag 7.2%. This alloy does not match any mentioned in "A Study of Alloys Suitable for Use as United States Coinage". The piece was later offered in Heritage 1/17 FUN sale.
It is not known if any of the other alloys mentioned still exist.
These dies are now used by the U.S. Mint to test various planchet sizes and alloys. They were used in 1999 to test the new "brass" planchet used on the Sacagawea dollar. To view one of these trial pieces, click here.
A waffle cancelled example appeared in Heritage 4/14 sale.
An obverse die for this pattern is also in private hands and is shown below courtesy of Mike Byers.