Most stamps between Scott #79 and Scott #144 are grilled.
To summarize, the USPOD (as it was known at the time) was concerned about the reuse of postage stamps. Cancels could be cleaned and the stamps would be reused, resulting in a loss of revenue to the USPOD. Several ideas to prevent the reuse of stamps were presented. Charles F. Steele’s idea of applying a grill to a stamp is the only idea that was actually used on issued stamps.
The grill was created by a metal printing plate that had an arrangement of tiny ridges. The ridges were meant to gently break the paper fibers so that the cancellation ink would not just “lay” across the top of the stamp where it could possibly be removed. Instead, the ink was meant to soak into the stamp paper due to the broken paper fibers. This was meant to make removing a cancellation much more difficult.
Several different grill patterns were used on stamps as the USPOD experimented with patterns that would not make the stamps too brittle to separate (such as the A grill) versus the smaller grills (such as the F grill) that maybe didn’t provide sufficient coverage on the stamp.
In my experience, many grills are poorly applied. Because these were metal printing plates with ridges, the ridges would wear over time. Meaning the ridges were not as strong. Any variance in the pressure used to produce the grills means that the grill was lightly applied.
You measure the grill from the back of the stamp. Either you count the number of grill points (which is what I usually do) or you measure the number of millimeters from edge to edge of the grill. When you’re dealing with weak or incomplete grills though, sometimes it is very difficult to distinguish which grill it is.
Here are some of my recommendations for identifying which grill you have.
Unless the stamp is mint with at least some of its gum, make sure the back of the stamp is clean. Remove any old hinges. Soak, dry, and press the stamp to remove dirt or anything else that could interfere with observing the grill pattern. This works best for mint stamps that have no gum or used stamps. If your stamp has any traces of original gum though, I suggest not removing the gum because that will take away from its value.
Now that the back of the stamp is nice and clean, can you distinguish the grill pattern? If not, carry on with the next step.
Lay the stamp face down on a hard surface such as a table. Place a thin sheet of paper on top of the stamp. Onion skin paper is extremely thin and usually works best. Do not use the point of a pencil. You risk ripping the paper and damaging the stamp. Take the side of the pencil. Gently rub the lead side over the paper. The ridges of the grill on the stamp will begin to make a pattern on the paper. Cover the entire stamp to make sure you have a complete impression of the grill.
The stamp remains clean and undamaged. Now you have a sheet of paper with the impression of a grill that you can attempt to measure.
This technique should work in most cases. Unfortunately, there are going to be times when the grill pattern is too weak or incomplete to make a firm determination. Or the stamp itself may be, say, torn or thinned and the damaged area falls within the grill pattern. In these cases, it’s probably safer to assume that you have the less expensive copy.
I hope these tips help make it easier for you to determine which grilled stamp you have.