Yours is a good question, and, perhaps we can get an answer which we can then share at our humble site. Your are very correct in assuming that what was penned by Mr. Chan is more than simply "Charlie Chan," which amounts to three characters (that are seen at our Entrance page among the opening text). It is probably something to the effect of a cordial greeting and best wishes. I also suspect that the actual writing may have been done by Keye Luke.
I'd have to look to be sure but my bet's on Luke doing the picture of his doing the canoe with the girl (Bo Ling, who was also # 1 daughter in Black Camel) in Shanghai.
She's also in another sequel/remake: Calling Philo Vance, an entry that was The Kennel Club set against WWII with airplane plans instead of Chinese art at its core. SHe was the housekeeper replacing James Lee, the original's houseboy.