Interestingly Yamanaka won the title in August 2017 by beating Tatsuya Fukuhara (19-6-6, 7) [福原 辰弥] with a narrow decision, Fukuhara had himself beaten Calleros, with another close decision, to win the title in February 2017.
Other than the win over Fukuhara there isn't a lot on Yamanaka's record, who won the OPBF title in November 2016, with a decision over Merlito Sabillo in pretty much his only other win of real note. That however is a diservice to Yamanaka who is a tough, exciting and energetic fighter who comes to the ring able to have a war, and come out on top. He's not a puncher but he is a talented fight who can box or brawl.
Calleros came up short against Fukuhara last February, but certainly won over Japanese fans by having a very exciting and fan friendly style. Since his loss to Fukuhara he has gone 3-0 (2) and bounced back with wins on the Mexican scene, including a decision win over former IBF Minimumweight champion Mario Rodriguez.
At the moment the bout hasn't been spoke about in the Japanese press but with a major source like ESPN talking about the bout, and getting quotes from Calleros about the contest, it looks like it's now a done deal and will be announced in the coming weeks.
Original ESPN article - http://espndeportes.espn.com/boxeo/nota/_/id/3872115/mexicanos-ganigan-lopez-y-moises-calleros-a-japon-por-titulos
(Image courtesy of boxmob.jp)