I can see the resemblance to Camilla.
It's actually quite possible that Charles got her preggers, she was packed off to the country, had the baby and then gave him up for adoption, and never told Charles anything about it.
If you think about it, back then, she could never have predicted in her wildest dreams that she'd end up married to him in later life. Back then, she would have just had the baby in secret and done a closed adoption, like they all were back then, and got on with her life. She wouldn't have even needed to put a father on the birth certificate. He could indeed be their son and Charles hasn't a clue.
People used to have babies in secret all the time. My elderly MIL told me a secret about a friend of hers, and I wish she hadn't. She's friends with a married couple well into their eighties who had been together since university. When the woman was 18, she got pregnant, had the baby, and gave it up for adoption. The couple stayed together, got married, and had three more children. The woman died a few years ago. Whether they ever told their three adult children that they have a full sibling, I have no idea. I have that information and I wish I didn't.
Point is, back then, it really wouldn't have been unusual for a well-to-do young unmarried woman to get pregnant and then have the baby and give it up in secret. I can absolutely see Camilla doing that. But if she did, I bet she never told Charles.