Some Chiyoda Kogaku (aka Chiyoko) cameras of the 1947 to 1949 era featured a blue ⓒ symbol as part of the lens designation like in "ⓒ Super Rokkor", e.g. on the Minolta 35 or the Minolta Memo. It was used to indicate a (single) coated optics rather than any copyright. Similar engravings can be found also on lenses of other manufacturers, e.g. some Olympus Zuiko lenses carry a red-colored "Zuiko C." designation indicating coated optics.
This symbol was widely used by the Cruver manufacturing company on their plastic recognition models that were produced during World War II.