I had a similar story with my own father.
I'm 33 he would be 76 but he sadly died 10 years ago
My dad was born in 1944, I was bone in 1986
My mum is 13 years younger than my dad, they met when she was early twenties, him mid 30's
It's obvious as an adult it's likely he had relationships before my mother but not brought up/ water under the bridge.
I was 13 when I overheard a conversation between my mum and dad about his first wife and his son.
I asked my mum and she said she didn't really know the details just that he had been married and had a son. That is totally her personality btw she wouldn't be bothered by his history.
He was just 20 when his son was born.
I never mentioned it to my dad EVER
Spoke about it with my mum once or twice she just said he was olde when she met him and he had a past.
I was surprised he would have a son out there and no contact with him but I didn't ask questions.
So...
A few years after he died I'm back living at home (divorced) and a letter arrives in dad's name.
It's from a woman who wants to know how he is and if he would like to know about how 'sons name' is.
My mum didn't really catch on to who wrote it and showed it to me saying she thought it was from his sisters friend.
I put two and two together and told her what she thought and she said 'oh' and put it in the bin.
So...
I take it out take a photo and put it back in the bin
A couple of weeks later I ring the number. The letter had upset my mum and I didn't want her sending more letters and thought she should know he had died not just ignored her letter.
Our conversation was so interesting I found out what had happened and that my dad had married young because girlfriend was pregnant. They coped ok but weren't happy. My dad left (mutual decision) his son was quite young and his ex met someone new and fell pregnant. Ex and new partner got married and NEW PARTNER ADOPTED MY DADS SON
The boy was raised not knowing but the ex told me on the phone his aunt dropped hints at a party so he might now on some level.
Her new husband raised both kids and sadly died about two years before she wrote the letter. She didn't know whether to tell her son or not about his bio dad.
We had a really good chat and I felt like I got answers and she got closure. My dad passing she thought there was no point to raise the issue as he could never meet my dad.
I told my mum about a year later as we saw my dads sister (who lives 300 miles away) and I told her about it all.
Long post but it's interesting for me.
I think this stuff probably happened a lot when there was still stigma about kids from divorce or born out of wedlock
It's a shame my dad never got to know his son but I think he thought it was in his sons best interests
All had happy lives.
It's the secrets that keep you up at night