I got this nice stripped grey Sheaffer, Triumph nib, lever filler, and it needs only a new sac. I am however at the end of my wits trying to remove the section. I have pulled in all possible directions with moderate pressure, put the pen in very hot water... Is this section a screw-in, or a friction?

As nearly all lever fillers, I think a Triumph lever filler uses a friction fit to secure the section in the barrel.
I don't have one in my collection so I am not sure for 100%. Richard Binder can surely confirm if a screw or friction fit is used. If you turn the section counterclockwise you can't do anything wrong for both cases. Note you should heat up the barrel/section fit area in 60°C water for at least one hour before attempting the removal of the section. This will allow gradually softening & weakening of the shellack.
I experienced that injecting some water internally in the barrel through the lever slot is very helpfull to reduce the shellack softening process time.
The removal of the section can be done gripping the sec tion using section pliers -covered with rubber on the gripping sections ! - while holding the barrel with a rubber gripping mat in the other hand.

That was very helpful. I will let you know how did it go!

Well, the hot water trick did not quite work, so I went to Home Depot and got a heat gun. The section came off and it is a friction fit.
I made another discovery. Heated plastic melts...
The section is not straight anymore... I hope this can be fixed, because the pen is really, truly beautiful...

Sorry to hear the barrel is deformed applying the heatgun.
This risk is in fact the main reason why I prefer a temperature controlled heating pot.
Is the barrel really partly melted or is there a torsional deformation caused by the torsion while rotating the section ? I think the latter problem could be restored by re-heating & carefull torsion in the opposite direction.
By the way, did you buy a temperature controlled heat gun ?

The deformation seemed to be more in the section than in the barrel. I reheated it, inserted it in the barrel and pushed in the opposite direction. Now, it seems ok .
I put a sac on it, and it works just fine. All in all, a very nice pen, with a smooth medium nib.
Well, the heat gun has a control with only two positions, low and high...