Thread Naming / Designation

Not really model engineering related but I suspect someone here may have the expertise or have come across this:
In modern motorhomes/caravans, the LPG regulators are mounted on the
vehicle, rather than the gas cylinder. A pig tail (rubber or stainless steel) is then used to connect to the LPG cylinder (or cylinders, possibly by some change over arrangement). Most people use Propane and the cylinder end tends to be a 'POL' connector but there are others, including a 21.8 LH thread generally known as the Butane connector, although it is used for Propane as well !
The other end, ie the regulator (end, we can ignore any change over arrangement), is most commonly known as a W20 although sometimes an M20.
I'm trying to find out want kind of thread the W20 is.
Looking on the 'net, it seems the W20 and M20 aren't the same but will 'mate' well enough, even for gas- the thread isn't part of the seal, that is via a washer (rubber I assume).
The M in M20 suggests a metric thread and while the W could, perhaps, suggest Whitworth, I thought the normal shorthand was BSW, plus I've never heard of a metric Whitworth thread.
The 'nut' part of the female fittings all, at least all that I have seen, have small notches in the edges.
Others, like myself, who have investigated this from the motorhome/caravan world seem to have lacked the expertise in threads etc to nail this mystery. While we can, and do, tend to simply buy the correct fittings etc, I'd like to get to the bottom of it.
Can anyone help, please?
(Things get even more complicated if you want to connect to an European gas cylinder as some have different connectors on the top but that is another can of worms. You can't get UK gas cylinders in Europe.)
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On 11/03/2019 10:36, Brian Reay wrote:

As always, you haven't got a clue.
Leave well alone, gas installations can kill, and not just you.
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On 03/11/19 10:36, Brian Reay wrote:

You could use a thread gauge to measure the pitch, then look it up in thread tables. Not much experience of the type, but probably some variant of bsp, gas thread or similar...
Chris

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On 13/03/19 00:22, Chris wrote:

Thank you for the suggestion. However, based on what I've gleaned from various sources so far, the differences between M20 and W20 are more subtle than pitch. The basic dimensions, if I can use that a crude collective term, for the key features of the thread- inc pitch and diameters seem to be very close. (The two 'mate' quite happily, certainly well enough so the part of the connector which seals is gas tight.) While a measurement with a vernier or micrometer would probably show any difference in the overall (major?) diameter, would a visual difference in the pitch be apparent? Others who have also researched this have mentioned angles, 5 and 7 degrees are mentioned, but without a context, the 2 degree difference could mean anything.
While, in practical terms, I (and others) can simply buy the appropriate fittings and they will work safely, it is just one of those things I'd like to get to the bottom of. It isn't something you need to work on often, more something that crops up when you need to replace a hose etc at the end of its life (typically 10 years for a rubber one). My interest was stimulated as our new motorhome has a clever system which allows you to run the gas heating while driving. While I don't use it, I read up on the details etc and it includes some special, extra, fittings which cut off the gas if the hose rupture in an accident, plus there is a 'crash sensor' which cuts of the supply even if there is no rupture. (Normally, you simply turn off the gas at the cylinder(s) before driving, which I still do, not least as most of our trips are via the Channel Tunnel and they demand it.)
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uk.rec.models.engineering the following:

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