Ruger doesn't use a numeric or alphabetical "code" within the serial number to date it's production, like Browning & other makers - the entire model (Mark I, Mark II) & serial number is needed for a DOM.
As posted above, with those, the DOM can be found on Ruger.com under "product services" / "serial number history" /Model......etc.
Also as posted, a call to the Ruger Cust Service Dept will give you the actual shipping date/comfig - both of which may differ from what you may think.
The "plum" coloration is desireable to Ruger collectors, and probably makes that rifle a tang-safety Mark I.
Depending upon what exact M-77 config ( 'R'/"RSI"/"RL"/"DLZ"/etc), it would be worth every bit of the asking, and more.
For instance - I have sold two, very rare DLZ's (blued/brown lamo stocked ultra-lights) for $750 (each) more than two years ago - and haven't seen any since.
.