Just had a look with a loupe and the red is definitely frit, can see that it is granular and it has a distinct edge. From the inside it goes clear glass, white characters, red.
In my imagination, a round stencil gives the red shape in powder on the marver another deeper stencil for the letters. First the letters are picked up on the hot bubble then the red circle, blowing/shaping is finished, add handle etc... Simple, unless you actually have to do it yourself!