It takes very special people to adopt a child as their own...successfully. You are one of those special families in my opinion.

 Mom and dad, had no biological children of their own. It was not their choice, but as they put it, God's plan for them. I was their first adoption of three total. It was never a secret that we were adopted, but it was also made very clear that that fact was completely null. As far as mom and dad were concerned, we were "their" children.

 I have held that value true throughout my life. I have never sought, and never will seek, contact with, or knowledge of, my biological contributors. I also feel no ill toward them. It is my personal choice. My adopted little brother has met his biological mother and keeps some contact with her. My adopted little sister is seeking contact with her biologicals.

 Speaking from my point of view as an adopted child, I feel that it is important for the children (single or plural) to know who to call mom and dad without question. It is important for a young child to have that anchor point, and that the anchor is secure. It is important to know that it is ok to call the biologicals by their name(s), IMO, so as to reinforce the anchor point known as mom and dad.

 Of course, the early 60s were a lot different in all ways, including adoption law.