I expected the grouphead's heating to go around the head, but it stops halfway down the neck (thin red and blue lines).
I don't think it will give much heat.

The brewing water (thick red line) is collected in the neck and then brought to the sleeve via a thin pipe drilled at an angle.
The tip of the screwdriver extends into the brewing chamber.
From the sleeve it is distributed over the puck via 4 holes.

There is an Allen screw in the neck, where the brewing water enters the obliquely drilled pipe.
I had expected that to regulate the flow to the pf, but that is not the case.
It seems to be a possible connection between the brewing water from the HX and the heating water from the boiler.
I can't get it out.

Does anyone know what its use is?

Does anyone know what the function of this pin (?) / this hole is?
It is about 22mm deep, almost as deep as the collar of the group.
I was hoping it's some kind of clever relief feature, to prevent the PF sneeze, but it doesn't seem to have any connection with the chamber.
Perhaps a leftover from the manufacturing process?

That's a stop pin to keep portafilter from turning too far if the gasket is too worn or if you use an incompatible basket.

That's a very reasonable explanation, thank you.
(I could have made that up myself)

guijan12 wrote:It seems to be a possible connection between the brewing water from the HX and the heating water from the boiler.
I can't get it out.

Does anyone know what its use is?

Was this group also used in non-HX configurations? In that case having a connection from the dipper tube via the group back to the boiler could also work as a thermosyphon (though it already has a loop from the boiler and back).

Another option is that it just seals a hole that was needed to drill the thermosyphon loop (like with the screw on a E61 group).

That's interesting, Rob, and may well be the reason.
While searching for the right exploded view of this group, I see a lot of (at least) very similar dipper groups, from both Bezzera and Astoria.

I'm not sure about the drilling, since the thermosyphon loop has an opening in the side of the neck, that could have been used for drilling.