Bob, John,
John: I was wrong with the 22 mm. The correct french tube dimension is of course 28 mm, and nothing else.
The bike, a Stella from the 1960s, has brazed-on cable housing stop for the front derailleur on the down tube. Preferably, I would like to find a cable stop to put on the seat tube, below the derailleur. This will rule out the original Simplex double cable guide model 3606-L Bob mentions, however.
Thank you both for your help.
Mike Anderson
Gothenburg, Sweden