A recent tweet from Kimio Tanaka indicated that Ricoh has no plans for a 24x36 or 645 mirrorless camera. Mr. Tanaka said that he speaks on behalf of Ricoh and is not expressing his own opinion. Kimio Tanaka is a Japanese photographer who has Ricoh/Pentax insider information and has a good record of reporting relevant information.
In an interview with Nippon Camera magazine last year, Ricoh said that they have no plans for a mirrorless camera.
Flashback - Pentax had a "Digital K" mirrorless camera concept back in 1997:
Via PentaxRumors
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