See below for details
Detail shown below
The grill seen on the reverse of some stamps
Partial grills will always be considered to be H grills.
If your stamp has the a grill on the reverse you have quickly identified the stamp. It is the first printing of this stamp from the National Bank Note company and is #138
There are two types of grills, the H grill and the I grill, the I grill being far less common, and as a result has a higher value. The H Grill is #138 and the I grill is #138a.
Look for a semi circle drawn inside the lower right ball where it meets the rest of the design. Please refer to the image above.