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This stamp may have a rectangular grill pattern impressed into the back of the stamp. These patterns vary in their dimensions, each different dimension receive a classification. These are "A Grill' ''B grill' etc. There is one other color, lake, due to its rarity you will be unlikely to find.
The dimension of the grills are as follows
A grill: covers entire stamp
B grill: 18mm x 15mm, 22 x 18 points. Points up. (only 4 copies survive)
C grill: 13mm x 16mm,16-17 x 18-21 points. Points up
D Grill: 12mm x 14mm,15 x 17-18 points. Points down
E Grill: 11mm x 13mm, 14 points x 15-17 points. Points down (common)
F Grill: 9mm x 13mm, 11-12 points x 15-17 points. Points down (common)
Z Grill: 11mm x 14mm, 13-15 points x 18 points. Points down (very rare)
“Points Up” refers to the embossing being from the bottom of the stamp paper so that the raised pyramidal shapes are on the face side of the stamp, or pointing up. “Points Down” is exactly the opposite.