In article .uk,
"Ed Sirett" writes:

I've seen gas appliances which have some sort of internal proprietry couplings
Usully involving a machined ball-cup or cone-cone mating.

The Keston system kit comes with those for making the external
connections for water and gas. IIRC, it has a gas one inside too.
I think they're used where you want a joint which is suitable
for repeated dis/assembly but remains reliable.

I used to service my parents' Ideal Standard boiler (which was
replaced about 5 years ago at the age of 35 years old) and that
used them for the internal gas connections which you had to
disassemble each time you serviced it to get the burner assembly
out. They work very nicely for this purpose.

--
Andrew Gabriel