1970? I have comp books back to 1988. Call the central office of your MLS and see if they have comp books that far back. Perhaps census data for the market analysis. No real help here. I'm glad its not my assignment.
They must be establishing a basis for their value to calculate capital gains. WOW .. what an assignment. Our office has only been in place since 1973 ... good luck on this assignment and Im not sure 12,000 beaver pelts is enough with all due respect to Mr Miller. Hes the fur specialist though .. he used muskrat pelts for a reason I am sure.
If it's just a residential property then you could probably spend a day or two at the county recorders office collecting data for the period. You might also interview a few old timer Realtors to get some background on market conditions for the period. Perhaps you known an old appraiser who might have some general thoughts and ideas (like my dad) or TC.
It's a retrospective appraisal assignment and you need to read up on USPAP.
If it is 38 yr ago. Trust me - charge by the hour and tell them that at a minimum it will be $6000....you will put in the time to do this. You have to find very old Realtors - most of which are retired - and Realtors who have records. You will likely spend the first 3 days finding out who was in biz then and contacting the few survivors.
Call the water department; see if they maintain records of name change. I just did research and found records back to 1899 with name changes, but I was also looking back to 1968-1970 for data. If you see name changes you could find deed data if the deed host the consideration.