It's a Plate Heat Exchanger (PHE) RX-13 series of Hisaka Works.
I'll explain how to attach a gasket to the D plate.
Please also watch this video on the RX-10, RX-30, RX-50 series.

HISAKA WORKS, LTD. Heat Exchanger Division https://www.hisaka.co.jp/english/phe/