acidic paper t.54:vyp.1(1918),t.55:vyp.2(1919/1923)-t.56:vyp.2(1924) acidic paper pH pen test brittle covers t.55:vyp.2(1919/1923) brittle covers double fold test--breakage on second fold loose covers t.55:vyp.2(1919/1923) stained covers t.55:vyp.2(1919/1923)