There were three relatively recent articles in The Numismatist on these coins:

March 1985 beginning on page 466
August 1987 beginning on page 1633
January 2011 beginning on page 36

Regarding the Stone Mountain half, I found the source below very helpful when I was doing the (of course) Wikipedia article on it. I don’t recall if it was simply a reprint of what was in The Numismatist, or if more matter was added, but it was very good. The ANA library has it in their pamphlets file.

Hyder, William D.; Colbert, R.W. (1985). The Selling of the Stone Mountain half dollar. Colorado Springs, Col.: American Numismatic Association (pamphlet with reprint from March 1985 The Numismatist).

R.W. Colbert and I wrote an article, The Selling of the Stone Mountain Half Dollar  that appeared in the March 1985 Numismatist. We could not identify the G.L. and S.L. countermarks, but that was later solved by another researcher--Gold Lavalier or Silver Lavalier--coins in bezels awarded to the first and second place winners (young ladies) of the most sales of the halves in their areas. See Unraveling the Mystery of the Counterstamped Halves by A. Steve Deitert in the January 2011 Numismatist.

There are many other articles out there including Adna Wilde's censuses of known countermarked halves. These two articles were informed by original documents and brochures from the period. I suspect there are still gems of information waiting to be found.

I have been conducting research in hopes of writing a book around the 50 piece series of early commemorative coins. Last week a fellow Numismatist asked for information around Counterstamped Stone Mountain Half Dollars. Adna Wilde pretty much "wrote the book" around the counterstamps. Mr. Wilde's research can be found in The Numismatist, "Counterstamped Stone Mountain Half Dollars: Where Are They Today?" By Adna G. Wilde Jr., LM232, August 1987, p. 1633-1643. If this research can not be found online, the ANA can copy the pages for a small fee.

Hope this helps a fellow researcher,

Regarding the request for information about the research Arlie Slabaugh did on counterstamped Stone Mountain half-dollars, Whitman published his book that included a good description of the pieces. If this work has been overlooked I suggest it is worth pursuing. Arlie was a careful researcher and writer, as well as a good friend to all who knew him.

United States Commemorative Coinage. Arlie R. Slabaugh. Whitman Publishing 1962, and second edition 1975. Hardcover 6”x9”. The second edition contains some additional information on the counterstamped pieces.

Bill Shamhart is a dealer in New Jersey who has dealt in some of the Stone Mountain counterstamped halves.

There were three different medals issued in 1970 – a 70 mm Bronze with 500 made, a 70 mm .900 Silver with 1500 made and a 40 mm .900 silver with 1500 made.