Adult adoptee here, US, 1970s.
So, I was reading the letterbox thread which is also going at the moment but felt reluctant to post there. I feel like I need some place to express this though.
I found the idea that others knew more about me than I did really difficult growing up; definitely contributed, I believe, to a sense of paranoia!!!! As an adult, I was able to find out more but that is a question of luck. What you can find out depends on the state-by-state rules in the US. One of the people at the adoption charity, when we both concluded my BM didn't want contact, told me more than she should have as well. So, I feel like a previous poster in that many questions I had have been resolved.
I have to say, however, I am sceptical of how open adoption seems to work these days. i'm sure how well it works must vary from family to family but reading some of these threads on here, it often seems very intrusive and I wonder if the child grows up feeling like they have to manage the happiness of BOTH the adopted parents and the birth parents. That would be a nightmare. I already felt responsible for my adoptive parents happiness but if I'd received a letter like the OP got for her DC, as the child, I think it would have driven me round the bend.
I feel like there could well be a lot of pretending going on: I'm pretending I'm okay with you having given me up, I;'m pretending I'm okay with you contacting my DC, I'm pretending I'm okay with you haveing my birth DC...
I think having all books and records open from the age of like 16 to access is absolutely right; they should not be able to keep things from adoptees in the way they have tried to previously.
A friend whose husband is also adopted told me about her friend in the States who adopted (still with me??) and remained in contact, like practically on a weekly basis, with the couple who were teens at the time but are still together now. They are in a religious community and it was a private adoption. The couple puts pictures up on FB of them with the baby; very open they're related and the birth parents. I really feel for the adoptive parents. I can't even imagine the strength it would take to accommodate this.