Here's couple more straddle pane copies for you to enjoy:

While not as nicely centered as your example, this example shows the guideline at right.

This Scott 78 shows part of the stamp on the right side.

Again, you see the left edge of the adjacent stamp on the right of this straddle pane stamp.

Here's another example of the arrow, similar to the one cfrphoto showed. This is on Scott 215.