The biological dad knew, the other did not. When I found out, I asked the donor about it and he didn’t really confess. When I asked why, he worried about the feelings of the other. I guess I could see that. After non-donor passed, donor admitted it. He protected non-donor and I respected him for that.
I’ve told my story to a few people over the years and you’d be surprised how many said they had similar things happen in their family too. Remember that children out-of-wedlock was a BIG deal back when. It was largely frowned upon and pretty well hidden. Times sure are different now.
It sounds like you were fortunate for the donor to be discreet and respectful.