I'll go a little further with the explanation since a bit of history is involved.
When TFL was started about 11 years ago, Rich wanted to personalize the thread/post images. The standard vbulletin image for a thread in which you have posted was an envelope with the arrow like the one in the locked thread image. He made that into a post sight type of image. The vb image for a thread in which you hadn't posted was simply the envelope, he made that into a bullet hole image.
However, he didn't change the images for locked threads and that is why the arrow isn't a familiar device. If the original images hadn't been changed, you would have seen that arrow/envelope combo next to many, many threads (and it would have been in the legend).