https://reference.discogs.com/wiki/japanese-release-dates

In Japan, it is common for a release to have two dates listed on it. The one with an X~ next to it is when it is legal for the release to be rented. For music of foreign origin (Ⓨ), the rental prohibition is automatically at least one year. Sometimes, older releases are added with the rental date used as the release date,  since they would use a Latin letter in the place of a release year while the rental date would have a proper year number, leading to confusion (O = 1987, for example).

These dates are of some historical interest, I think we should be able to add one to a release without relying on annotations.