Not 1971, but 1979. Marlin used letters for 336's from 1946 to 1968 C-AD. From 1968 -1972 the first two numbers in the S/N was the last two numbers on the year of manufacture; I.E S/N 71xxxxxx is 1971. One exception is 1968 which uses both letters and numbers and are numbered both AD and 68. After 1968 subtract the first two numbers from 100 for 336 rifles manufactured up to about 2009/10 I.E 91xxxxxxx would be 2009 ; 93xxxxxx is 1997 It gets a little murky around that time in the transition to Remington. But your rifle is easy 100-21 is 79, thus 1979. BTW it's a beautiful rifle. I was asking about your S/N to determine if it was a waffle top. I stupidly passed up a two of them a couple of years ago at very good prices.
I am praying for a successful procedure. Someone should jump on this beautiful JM marked 336 C w/o the silly cross bolt safety added in the 80's.