Your "Letsekeen" is most likely an english writing of the german pronunciation of the town "Letschin", Brandenburg/Preussen, Germany and is situated east of Berlin. (http://www.letschin.de/cms/front_content.php)
The given name "Martin" with the surname "Fröhlich"/"Froehlich" is in general quite rare in that days. But in the town of Letschin south of Berlin/Germany still live some Fröhlich families. Some of their ancasters up to 1730 had been farmers with the given name "Martin".