Herman Miller Aeron Classic Black Mesh 3d01 Replacement Seat (Frame + Mesh) Size B – OEM-style complete replacement seat for size B. This is a complete seat assembly, which includes. ?? The plastic seat frame (seat pan). ?? The mesh already installed. ?? Pre-assembled unit (no stretching mesh yourself). This part fits the original Aeron model (pre-2016), not the newer Remastered version. Your chair is Classic if. It was made before 2016. It has the older tilt knobs (round style). It does NOT say “Remastered” on the label. What “Size B” Means. Size B = Medium (most common size). You can confirm by checking the back of your chair frame.. = Size A… = Size B…. = Size C. If you see two dots, you need Size B.