Nice google book search engine.

Billboard magazine 1980 (page 30):

http://books.google.pl/books?id=kSQEAAAAMBAJ&pg=PT29&dq=queen+mercury+billboard&hl=pl&ei=dvjrTJf6K5Df4gam_oSRAQ&sa=X&oi=book_result&ct=result&resnum=3&ved=0CDYQ6AEwAg#v=onepage&q=queen%20mercury%20billboard&f=false

Billboard magazine 1974 (page 21):

http://books.google.pl/books?id=awkEAAAAMBAJ&pg=PA21&dq=queen+mercury+billboard&hl=pl&ei=uPjrTMKaBtGC5AaexI3WAQ&sa=X&oi=book_result&ct=result&resnum=8&ved=0CEsQ6AEwBw#v=onepage&q=queen%20mercury%20billboard&f=false