Lots of info on different types of Verschraubung here:
https://www.ikz.de/ikz-praxis-archiv/p0305/030510.php#:~:tex...
It looks to me as if the Holländerverchraubung is nothing more or less than a "Konusverschraubung und [or] Verschraubung mit Flachdichtung [and 'Holländerverschraubung' is a convenient way of combing the two types under one name]: Die Überwurfmutter ist auf dem Rohr verschiebbar. Am Rohrende befindet sich entweder ein glatter Bund oder ein Konus. Bei der ersten Variante wird ein zusätzliches Dichtelement (Flachdichtung) eingelegt, bei der zweiten Art dichtet der Konus mit Gegenkonus die Verbindung. Bei beiden Varianten wird die Überwurfmutter auf das Gewinde am gegenüberliegenden Rohrstück geschraubt und angezogen, um die notwendige Kraft zum Abdichten aufzubringen."

If that is so, it is simply a 'union'.

"A union is a threaded fitting which allows the pipe work to be separated and reconnected without any horizontal movement in the pipe.
It can be a standalone pipe fitting connecting two pieces of pipe, or an integral part of another fitting (such as a ball valve) which allows it to be separated.
[If it is a flat-seal union] An O-ring seal sits within the union body, compressing when the union is tightened."

" Pipe Union is a type of fitting equipment designed in such a way to unite two pipes which can be detached without causing any deformation to the pipes. Any kind of small diameter piping connections requiring a positive seal and easy assembly as well as disassembly are made with the help of pipe union. In other words, the pipe unions can disjoint two pipes very easily. They are widely demanded in the pipe fitting market.
[…] A pipe union consists of three parts – a nut, a female end and a male end. To make an effectively working pipe fitting, both the male and female ends are sealed together with the help of nuts. A female threaded fitting has threads on the inside, while in the male-threaded fitting, the threads are on the outside."
https://www.thepipefittings.com/pipe-union.html#:~:text=A pi... [worth reading]

[I don't actually believe a union necessarily has only a male end and a female end, especially in these days of non-binary/gender liberation]

See image.



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Note added at 7 hrs (2024-02-24 00:16:37 GMT)
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Note added at 8 hrs (2024-02-24 00:41:27 GMT)
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Note that a Holländer or Holländerdichtung is — in the southern reaches of the German-speaking realm, at least — a flat seal or gasket. Traditionally such gaskets were those red fibre things that, by swelling when wetted, produced a tight seal. Maybe there's a connection with a waterlogged Holland … These days of course they are more often than not plastic (on cheap Chinese articles), nylon or similar, or rubber.

See image.

On the strength of that, then, a Holländerverschraubung might be the flat-seal type only, not the conical type, contrary to what Heco appear to be saying (see Discussion) with their "Unter dem Begriff Holländerverschraubung finden Sie in unserem Produktsortiment also alle flach dichtenden und konisch dichtenden Verschraubungen".




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Note added at 8 hrs (2024-02-24 00:44:35 GMT)
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Oops. Sorry for shouting. I haven't yet found a way to preview additions to an answer before posting and spot formatting errors, not in the old version or KudoZ and even less in the new version. In case Henry is listening ...

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Note added at 18 hrs (2024-02-24 10:38:35 GMT)
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