Adam, many thanks for your info, as always. I think Thomas Kopp is the person credited with introducing selenium red, isn't he? His company was the one that made the first red signal lights, I believe (could be wrong, of course).
My information regarding red Carnival is based on that given to me by (among others at Fenton) Fenton's Chief Chemist. So it is limited in its relevance to their pressed output.
I've got another little bit to add to this topic too - check out your red / amberina pieces with a UV (black) light. You'll notice that the amber parts glow! I took the following from a 1938 issue of the technical British journal "Glass" (described as "the recognised organ of the glass industry") that concentrated on the effects of UV on various compositions of glass. The main colouring agent that causes an orange glow under UV light is cadmium, usually in the form of cadmium sulphide. The journal reported that the fluorescence could vary from a citron yellow through to a dark red. Cadmium was usually added to the glass batch along with selenium to create red glass.
A further fascinating fact emerged - red to amberina glass will glow where the amber parts are. Selenium red with no amberina does not glow. To achieve the red glass the item is usually reheated to strike the glass. Until the moment of striking, the selenium/cadmium glass will react to UV light and glow. However, when the red colour appears, the fluorescence vanishes.
I have only tried the above with red and amberina Carnival (also the unusual colour called "Geraldine's Delight" that was made by Summit and...I think...Kemple before that. Someone will correct me if I am wrong). It's fascinating to see the orange glow. Try it on your comport, Barbara.