Richard wrote:
Set Square wrote:

In an earlier contribution to this discussion,
Richard wrote:

As others have said, so-called stainless steel pipe was used in the 60's and
70's when there was a copper shortage. The sizes will be 1/2" Imperial**
(virtually identical to 15mm) and 3/4" Imperial - whose OD is just slightly
less than 22mm. You can use 15mm compression fittings on the 1/2" stuff, and
22mm compression fittings with special imperial olives on the 3/4" stuff. I
wouldn't advise trying to use solder fittings on this pipe.

** Imperial pipe sizes refer to the nominal ID rather than the OD - which is
why 1/2" pipe is actually more than 1/2" in diameter - and the same for 3/4"

So, my 'magnets won't stick to stainless steel' test is flawed?

Yes.
Some stainless steel is magnetic, and some stainless becomes magnetic
after it is stressed.