I agree looks confusing, that's the year 2000.
My model of heat exchanger tube has 3 rings that seat in the radiator housing. One at each end and one in the middle. The ring/bushing of course also hold the tubes ... On removal
the center one seized to the housing and dragged all the way to one end...
Looking at the radiator design it seems the center bushing seals
between the 2 sides on the radiator housing to force the water
flow through the gaps in the tubes.... It makes sense.
But creates problems for maintenance
. On my starboard engine
I had no problems, of course.
Not sure why this unit was designed this way, and why such small gaps between tubes, really invites problems.
I am sure I have to get a new unit and further clean up seatings in housing.
As you say it seals between the sea and coolant just by the end bell housings ... Far from a good design, Thanks Volvo