OK, I took this down to clean it, and I noticed something interesting about the structure of it that I hadn't noticed before.
The interior of the bowl is flashed with opaque white glass, but the main body is formed of a layer of clear glass, and then the red and white cameo layers are placed on top of that clear glass. The cameo is then cut into those top two layers. The clear layer is fairly smoothly finished, unlike my Webb vases.
So - thin white opal flash inside, clear body glass, overlaid with red flash and white flash layers into which the cameo has been been cut. Very delicate engraving on the cameo.