Thanks a lot for sharing! My earliest findings for the Candy nick name were of 1935...
But when he already had it with 18 just starting his musician career, why shouldn't this already having been his obvious nick name when he was still at school? Just shortening his last name to something nice sounding may happen already early in school, it happened also to me when I was about 10 years old. So he may simply have sticked to this.
Here is the watch engraving:
And here a nice autograph, the "C" style is even similar...
Three possibilities here, 1. Coincidence, 2. he copied the watch style when signing, 3. the watch was engraved later copying his signature a little bit.