Although the 1970S half dollar above was struck from proof dies, it is not an aluminum die trial. In 1970, the San Francisco produced 3 denominations of coins in aluminum for the country of Nepal. Their thickness is about the same as a regular U.S. dime as is the half dollar above. Error dealer Jon Sullivan discovered that some of these were struck over a Missouri token issued by Shell Oil Co.
Regardless of the planchet source, these pieces were clandestinely struck. During the late 1960s and early 1970s, many suspect San Francisco Mint errors and pieces such as these were made and smuggled out to the collecting fraternity.
Eisenhower dollars on these same half dollar diameter aluminum planchets and/or tokens are also known, one of which, from the Dr. A.K. Berry collection, is illustrated in the 7th edition of Judd.
Photo courtesy of Mike Byers.