I've never had any wine from 1969, so can't comment from personal experience.
However, I do have experience of having had colheita wines from "poor" years which I have been very surprised and impressed by. My suggestion would be to continue to do your research and see if you can dig out any tasting notes from anywhere on the wines that you have found. Someone, somewhere must have drunk these and recorded their thoughts.
If not, I look forward to you arranging a horizontal offline tasting of colheita wines from 1969
Glad to know these are at the top end of pricing. I know Richard Mayson gave the vintage a 1 star rating and only comments on the Taylors Vargellas. I'll just wait and see till one of these comes up at a sensible price.