Comiket 74 special.
Our horizons continue to expand beyond "rare character songs" and into "rare CDs in general", but this one is two different kinds of unexplored territory - a Comiket 74 CD and a Radio CD. The theme song to the radio show hosted (and still ongoing) by Mai Goto, voice of Mika Nogizaka, is contained within, along with a special radio show. The title was translated as best as possible to convey the pun.
Nogizaka Haruka's music isn't done yet. Two drama CDs, each with a song by the group N's created for the series, are scheduled for the next two months. If they're overlooked, you can count on Celonius28 to look right at them. Onward!