From what I found the casting numbers on case are GM 465452, 6273212 or 3901127 to indicate the SM465.
Also top shift cover should read GM390113.
From a Muncie transmission website I found I interpreted the 9312 as;
9=1969, 3-3rd month MARCH, 12 is 12th day
Just my read on those 4 numbers and how other transmissions were broken down on that website. There is nothing specific on the 465 there.
https://www.dmetransmissions.com/casecodes