Not as rare as lilacs but rarer than seals or blues. Seals are the dominant color, which is why there are so many, then blue is a dilute of seal. Chocolate is the recessive form of seal (so you need two recessive genes to get it) and lilac is the dilute of the recessive so the rarest to get.
"Why do you think they might want her back if I ever found the cattery? She's spayed and has been with me so long that we are so bonded I feel like a Siamese twin. "
Because some cat breeders are freaks. Sorry, there just isn't any other way to describe it (unless you want to use the word "nazi") but not all of them are that way and legally, I think they would have a very hard time trying to get back after all these years so you might be safe.
"She showed up when I was home from a major operation and bedridden for a month. She was about 12-16 weeks old and her hair was so thin you could see her skin. She spent the month in bed with me and policed the visiting nurses, examining everything they brought and making sure they did no harm."
Oh, poor kitty! Looks like you did a wonderful job with her. She looks just beautiful.
"I have seen sealpoints who get very dark, nearly losing their points, but she has stayed pretty white and if you look closely, her ear points at 5 yo are not quite touching (something you or another mentioned here)."
Chocolates are supposed to stay white, as are lilacs though some shading is allowed with age and truth is some of those colors shade pretty young anyway. Coat color on Siamese is heat dependent. If your cat was kept in a cold environment, you would notice her coat darken a lot more. That it is still pretty white at 5 years is a hallmark of good breeding in chocolates, one reason why I think her breeder specialized in them.
"Thank you for all your information. This is fascinating and proves I no little about the breed."
Siamese are one of the easiest breeds to learn about in part because they've been around for a very long time and their genetics are very simple if you stick with the four original colors - one reason I prefer breeding them, I would hate to sort out tabbies and ticking and who's masking what...
I'm going to take your advice and not try to find the breeder. When I ran into you and AnAmericanMother and realized you knew a lot it, got my hopes up that the mystery could be solved. Sam amazes and makes me laugh every day and I could never let her go. She's got me wrapped around her little finger to the point my vacations aren't as long as they used to be.