CHRISM:
>>>I am `Guessing` that FIRE acting upon Solder is the reason that You state
that Solder is NOT allowed on Gas Pipework in the U.S.A. ??<<<
I would have to guess also, I think it might be a hold-over from the days that we used 50/50 lead based solder which has a low melting point.
>>>So I am assuming that the Gas
Pipework in the U.S. [Domestic] is NOT Mild Steel / Screwed Joints<<<
We use black steel and galvanized, we also use copper tube but the joints have to be flared.
I have one thing I disagree with:
>>>Flux onto PIPE ONLY - NOT into the Fittings - as Assembly then causes Flux to enter into
the Pipework - unnecessarily, - When putting the Pipe into the Fitting - TWIST to spread the
Flux around the Fitting.<<<
This is a training problem with the individual that refuses to follow the directions which tell you to use an MINIMAL amount of flux (I've worked with so-called plumbers that PACK the fitting with paste) that causes major problem with flushometer valves; it's a good thing that commercial lines have to cleaned before the structure can be turned over to the owner.
Not fluxing the insde of a 6in copper fitting is going to cause you a lot of headache!
My answers are only general because every town, city, county and state may have a different idea
have a good one.