The Happy Slab is a neighbor of Batman Rock (also known as Chief Um), located a short distance to the west of the big chief. It appears in many of the same sequences as Batman Rock, albeit less conspicuously. The screen shot above, from Stagecoach (1939), shows the proximity of Batman Rock/Chief Um, a portion of which is visible at the top left of the screen, to the Happy Slab, which appears directly behind and above the driver of the stagecoach. You may be able to make out the "smile" that gives the Happy Slab its name, although the shot isn't very clear. (Click on the photo for a larger view.) The Slab faces to the right, and also has a little "eye."
You can get a better look at Batman Rock here.
Believe it or not, this is the Happy Slab today. All that's left is a little less than the bottom half. Condos have been built around it, and like Batman Rock, it sits beside a parking area. My guess is that as part of the condo construction project someone decided they had to blow off the top half of the Happy Slab because of concerns that the relatively vertical rock, which already had a lean to it, might be unstable and would be unsafe in close proximity to housing. It's just a theory.
If you look closely you can see that what is now the top of the rock doesn't have the dark coloration that's seen on the rest of the Happy Slab, which illustrates that the top hasn't been exposed to the elements for nearly as long as the rest. You may also notice that the rocks above and to the left of the Happy Slab also appear in the "Stagecoach" shot.