You were right, so they were the vans of the old seals. I heated things up a bit with the burner, after which they were demountable. Not easy yet, but it worked.
I find the head unreliable, it is a second hand from the Netherlands. Two oil penetrations had been drilled with the idea of making them larger. But after that it was one big hole. I have turned and mounted new buses.
A camshaft holder there had worn a stud thread. In addition, all hydraulic lifters were stuck, or did not move smoothly.
The camshaft was well worn, I replaced it with mechanical lifters. But a strange noise continued to be heard at the head and finally the head bolts became loose.
So I have now decided to fit a new head complete with hydraulic lifters, new camshaft and valves. So the problem wasn't just the valve stem seals, but it helped me over the threshold to get it right now.