Does anyone know how the quality-standard is
of Conn mouthpieces ?
I'm looking for a spare mouthpiece for my
Farkes MC and saw a Conn type H2 new in the box
for a reasonable price.
The seller claims it's a 'standard' type but
thinking about the matter I believe that there
is no such thing as a standard mouthpiece; it
all depends on the person who's using it.

Conn mouthpieces are acceptable. No known excessive complaints about them.
If you are looking for a 'spare' I would suggest getting the same kind of mouthpiece you are playing on. That being said, I think you might like the Conn H2.

Yes, the Conn H2 is a pretty 'standard' mouthpice. The cup is deeper than your Farcus MC. An advantage to this is you have more room to adjust intonation issues.

There is/are "Standard" mouthpices. Farcus MC, Farcus MDC, King H2, etc.
The Conn H2 is almost the same as the King H2 and is a fabulous all around mouthpiece. It will serve you well for many years.

Thanks for your reply ! So if the Conn H2
has a slightly deeper cup then a Farkas MC is it
then more like a Farkas MDC ?

Yes.