Earlier, John Becker recommended:
When you get a gauge, I think you will find the top/bottom at 11, and the sides at 10.5, thus Scott 632. In the meantime, if you have a '38 Presidential stamp handy, it is also 11 x 10.5 and can be used for comparison.
We can also help you here. Just take your stamp, along with any of the 1938 Presidential series (Scott 803 through 831 only), and scan them at the same time, and post. Then the knowledgeable people will use their software to show you how to easily compare side to side, top to bottom, etc.