@fist, that "heavy" mintmark might be the result of slight wear on the die used to strike this coin. It could also just be some circulation flattening as your coin has been well-loved over the past 46 or so years.
"If you climb a good tree, you get a push." -----Ghanaian proverb
"The danger we all now face is distinguishing between what is authentic and what is performed." -----King Adz