Father - Non availability of Marriage certificate

[Archum]

My parents were married in 1971 and did not register their marriage. At the interview, we were asked for "non availability of marriage certificate" document. When we contacted the marriage registrar office to provide the sample letter on immihelp, they said they cannot search the thousands of paper documents for approx. 5 year period and provide the letter, which is reasonable. We have the 2 affidavits from my uncles who witnessed the marriage, though not sure what do to for the official non availability letter.
Now we are not sure, how to get the letter.

[Vic99]

Hi Archum,

We're you able to obtain the non-availability of marriage certificate? How did your process go. Would be helpful to get some details as I'm going through the same problem.
Did you get GC for your father?