Not sure if any vendors in U.S. selling these indicators/sighters. Interesting concept for nymphing on a fairly short line and not over weighted nymphs. Just providing the visuals.

Allegedly more sensitive than traditional floating strike indicators, these sighters-indicators are just in the water or just out and within range to detect subtle takes. The nymph(s) cannot be too heavy. Notice above that one sighter has o rings and one has just the loops.