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 #135
There are two types of grills, the H grill and the I grill, the H Grill is #135 and the I grill is #135a.
The most reliable method of separating the National Bank's #135 or #146 from the Continental Bank's #157 is by the color. The former showing a noticeable reddish hue to the brown.
THE SECRET MARK
Look for a very slight dot of color where the ball joins the curve of the label border
Please refer to the image above.
I have not included the secret market in the identification as it is usually absent