Should be 1973. On most Marlins made from 1970 to the present you subtract the 1st 2 digits of the SN from 100 to get the date of manufacture. There were special runs of rifles and some of the 1895's that do not fit this formula however.
I was cleaning my Marlin 336 30-30 today and I got to wondering when I bought it. I bought it new at a K-MART store many, many years ago. Damned if I could remember. Then I thought about the Marlin forum on the fire here. I checked in and what do I see...your formula here. So using your formula, with a serial number of 231176**, my date of manufacture would have been 1977? That sounds about the right time I would have bought it. Thanks so much for your info. It's a bitch when the memory starts to go.