Catherineholm? or Krenit?  

Most
likely not Krenit
I could be wrong, but think they're always matte black on the outside.
Cathrineholm are usually patterned..and marked ?

What you said
But I don't know if CH made plain and if Krenits are sometimes not matte.

I've
never seen Cathrineholm in plain, or Krenit without matte black on the outside
But that doesn't mean they don'r exist

This person thinks these are Krenit
and they look to be (and I've seen them) shiny on the outside.
Still don't know.

Emalox, perhaps?
Emalox, perhaps?

Huuummm, I Don't think so....
Huuummm, I Don't think so. Emalox usually has an anodized aspect.
Kaj Franck for Finel perhaps?

Some kaj franck for finel that looked like this
.
I was just CHECKing to see if my tread had wasted away!
This is Kaj Franck for Finel, not marked (it had a foil label)
The weight and enamel thickness look similar, but not the shape and color.
Thanks for your brainpower...
.
http://www.worthpoint.com/worthopedia/vintage-kaj-franck-yellow-finel-pi...

Hmmm...
I have seen smiliar ones with the outside coated in black enamel and stamped. "Made in Yugoslavia" with size stamps of 15, 20, and 25. The only different with the ones I found, the rim is not black other than that they look identical.
I'm having problems adding photos, but you could Google it and see.

In
the time since this thread I've had a Catherinholm bowl in a plain colour, with steel exterior.

Pans...
So I started looking around and found a blog or two talking about these.
http://sellingvintage.blogspot.com/2009/11/sizzling-servers.html
http://www.etsy.com/listing/30941931/bright-red-round-enamelware-serving...

.
I have two Catherineholm oval platters. Stainless exterior and yellow/black
interior enamel. No pattern. The markings are tiny and need a film loop
to read. That is a lovely bowl!

I also have a Catherineholm...
I also have a Catherineholm small red plate with a plain stainless exterior, with markings under a Knoll sticker.