Actually, a MKI from 1971, will be a digit prefixed gun, and w/o any special markings,, according to the RENE Guide,, will be worth about $320. A box will add about $50.
And as mentioned,, the devil is in the DETAILS.
US marked, "D" marked, or any other minor feature can add to the value. (Possibly a tapered bolt?) The serial number is part of the key in helping determine values. If a person is afraid of posting a serial,, you can still share enough of it to help. 11-123XXD is a way to do it. And as noted,, pictures,, posted using a picture hosting site,, help determine the condition.