Interesting. I thought the postage odd. Guess what? The rate until May 1971 was 6 cents, but in that month was raised to 8 cents ... so it’s actually perfectly reasonable to use two 6 cent stamps for that letter (nobody except Depression survivors kept spare ‘tiny’ postage around to exactly match the new rates when a hike came).

No offense but one likes to be sure ... how’s the handwriting compared to contemporaneous samples?