Coleman:
I had the same mystery after I bought my 1980 Cherubini Hunter 36 in 2007. My motor is 2QM20 which as yours also converted by Yanmar to anti-freeze cooling with the external Sen-Dure exchanger circuit. *
Using a small mirror taped to a stick (how high tech!) so I could "see" into the small gap between the engine compartment siding and the engine, I eventually found the name plate located on the right hand side of the engine block (as looking from the cabin towards the stern) ... located underneath the starter motor. Then I was able to maneuver a camera (had to a real digital camera in the days before every smart phone had one.) Attached is the photo I took.
I agree with Jim Legere that the service manual makes no reference. But looking at the opening chapter diagrams my official Yanmar 2QM20 and 3QM30 manual, the 3QM diagram has a rectangle on the engine block under the starter motor. The diagram shows no details of it. Nor is it identified as a "part". But a chance this might be the name-plate?
Happy hunting!
(* If you want more info on Yanmar's Sen-Dure mod, let me know. I can direct you to previous posts five or so years ago.)