This is a 3M Anti-Glare Filter AG215W9B for a 21.5-inch monitor. The box has been opened, but the filter is still in sealed plastic wrap.

From the manufacurer:
Highlights
  • Diffuses light so your screen is easier to see in challenging and changing indoor lighting conditions, brightly lit indoor areas and near windows
  • Matte surface helps reduce glare and fingerprints

Details
  • Anti-glare: Yes
  • Aspect Ratio: 0.672916666666667
  • Blue Light Reduction: No
  • Brands: 3M™
  • Catalog Number: AG215W9B
  • Device Type: Monitors
  • Display Diagonal: 21.5 in
  • Display Diagonal (Imperial): 21.5 in
  • Display Diagonal (Metric): 546 mm
  • Factory ISO Certification: No ISO Certification information available
  • Film Classification: Anti-Glare
  • Filter Dimensions (width x height): 18.77 in x 10.56 in
  • Filter Dimensions - Metric (width x height): 477 mm x 268 mm
  • For Device with Raised Bezel: Yes
  • Orientation: Landscape
  • Reversible: Matte-to-Matte
  • Screen Attachment: Adhesive Strips, Slide Mounts
  • Touch Compatibility: Yes
  • Part Number: AG215W9B
  • 3M ID: 7100029120
  • UPC: 00051128831076