This one seems to lead to Couplet 35 of the Key to Genera of Perlodidae Nymphs and the genus Isoperla, but I'm skeptical that's correct based on the general look. I need to get it under the microscope to review several choices in the key, and it'll probably end up a different Perlodidae.
Radial sector: The radial sector (denoted Rs) is a group of longitudinal wing veins that normally branch posteriorly off the radius, but in mayflies usually appears detached at the base, looking like a separate vein rather than a branch. In some mayflies, especially in hind wings, it may even branch off of vein MA instead. It may have up to four branches (denoted R2 through R5) reaching the wingtip, with intercalaries in between. Vein Rs and all its attached branches are furrow veins (–), but their intercalaries can be ridge veins.
These other words reference the same concept: vein Rs, Rs vein, Rs.
