This is just a single "E" string, Original Flexocor.
Pirastro Original Flexocor (dark purple winding) rope core steel strings are especially suitable for arco. Original Flexocor offers brilliant sound, while providing short sustain. They can be a very good solution for large or inherently dark-sounding orchestral basses. If your bass is neither large nor usually dark, and you prefer brilliance without excessive edge to the sound, consider Flexocor Deluxe (red/white winding), and if you prefer a warmer sound with cleaner bow changes and string crossings, look into Flexocor (just Flexocor - maroon winding).
What strings are right for your playing style and your bass? Try our INTERACTIVE BASS STRINGS CHOOSER