Many thanks for your reply, Ray.
This is engine is a bit larger, produces around 270-275 HP. I wonder
what fuel do people use on all those gas engines around 1988-89, where
unleaded started to become common. THis 350ci block was in production
from 1987 to 1995 (Mercruiser site). Maybe some serials number have
indeed the hardened exhaust valve seats, and some not. This is what I
am trying to find out...

Greatings,
Dimitris.