Thanks for the replies.
I’ve done some homework in the meantime.
I have no doubt that you are right when it comes to the B18. As for the B20, however, I have come to the conclusion that some assemblers (the majority?) must have used a different routing.
I did some scratching around in my old-parts bin and managed to retrieve the old hoses. These are the original, factory-fitted ones. The pic below labelled “WRONG?” shows the way they were connected at the time that I replaced them. I am pretty sure that is also the way they were connected when the car was new. The pic labelled “RIGHT?” shows another way in which it would be possible to connect the hoses.
I trawled through some more pictures of engine bays on the Amazonpictures website. Of the 17 found which showed the configuration reasonably well (all 1970 and 1969 models), 12 have it the “WRONG” way.
The drawings in the GCP catalog are not helpful in resolving the issue. For the B20 they simply copied the drawing used for the B18 (including the illustrator’s error pointed out by Derek). However, the parts numbers they give for the B20 hoses do differ from those for the B18.
If you look at the CVI catalog:
http://www.cvi-automotive.se/en/arti...-heater-system
and cross-reference the parts numbers with those in the GCP catalog, both CVI and GCP appear to base their data on the “RIGHT” configuration (heater core first and then the valve, for both B18 and B20).
Another difference not apparent from the GCP drawings: The free end of the elbow fitting is shown to point in a NNW direction (if you stand in front of the car looking towards the rear). That may be the case for the B18, but in my B20 (and I don’t know if that is true for all B20’s) it points in a NNE direction. Use of “Hose 3” in the “RIGHT” configuration would be rather awkward if the elbow end were to be pointing NNW. I have an idea that this was done in the B20 to allow for some more movement between the engine and the firewall with “Hose 3” in place (and in the “RIGHT” configuration).
On balance I think that “WRONG?” is indeed wrong and “RIGHT?” right.
Whatever it is, it is clearly not mission-critical. I’ll take Ron’s advice and put the whole thing on the back-burner.
The thread can be closed.
PS: I agree with Ron's view about the core temperature when the valve is closed - ambient.