Before his death in 2003, the Collectors Club of New York declared him, in 1996, to be the "outstanding philatelist of the last half of the twentieth century."
He was also an expert in early American postal history, and was responsible for rewriting large sections of the Scott Standard Postage Stamp Catalog with regard to United States postmaster provisionals, locals and carriers.
Among other work he dedicated to the subject, he was editor of the Hoover Brothers Precancel Town List.
He signed the Roll of Distinguished Philatelists in 1964, and received the Smithsonian Institution’s Philatelic Achievement award.
He was named a Distinguished Philatelist by the United States Philatelic Classics Society in 2002, elevated to the American Philatelic Society Hall of Fame in 2004 The sale of his German States stamp collection, from 1985 to 2000, was so extensive that it required eighteen auctions by Köhler to complete its sale.