I think it’d probably be most elegant to call them 11-18 and display that in the bottom left corner. I wouldn’t prefix (something like S1), because if you did that, then you can’t use numbers to remotely call the shadow script (Teletype doesn’t have a way to concat a letter and number into a “string”, at least that I know of).
I think it’s mentally simpler to continue to use the SCRIPT op to execute these, rather than introducing a new operator like SSCRIPT.
Number 11-18 and utilizing the SCRIPT operator allows you to do cool things like
SCRIPT ADD A B, where A could be 1-8, and B could be a toggle of 0 and 10, allowing easy A/B’ing of the Script and its requisite Shadow Script
From a usability perspective, A teletype user knows to look at the bottom left corner to see where they are.
This would be sufficient, and in my opinion, less aggressive and distracting than say, inverting the colors. In my experience, the best UX decisions tend towards consistency rather than new concepts.