Thanks!
"Acceptable variants include:
- The letter c with a parenthesis over the top
- The letter c with a parenthesis under the bottom
- (c
- c)
- (c)
- The letter c with an unenclosed circle around it."
That sounds like there was some kind of dispute once about a hand-written © where the writer didn't complete the circle, and the copyright office is covering all bases here!
Interesting that [c] is not acceptable.