Separating the Dark Olive-Green 2172 from the Light Olive-Green 2172b can be a challenge at times especially if the 2172 is lightly printed. One simple method is to view the stamp under LW UV (fluorescent black light). The ink on the 2172 will appear olive green while the ink on the 2172b will appear black. The image below was taken under LW UV. The stamp on the left is 2172 while the right is 2172b.