Frank: thanks very much for Your input!
BTW: this is a Valvo EL519 not a 6550.
Most of it does not look like sharp spots (though there are some "spots" which are even independend of the rest of the discoloration and therefore I would call them spots) but rather as areas which seem to grow from there where the circular supporting frames touch the glass envelope as You said.
To me it looks like kind of a crystalline structure.
Don't bin your tube yet !
If I got You right these "spots"/areas show up only when the tube was used ?
This tube at the pic is pretended to be a NOS tube! Can this be?
Unless really old and used tubes I never have seen a NOS tube with this brownish stuff.
Is there a particular designation for this kind of effect?