Ah-ha... this one I have. That there is one half of a picture frame clamp jig.
Here's a picture of a full clamp (with a different working mechanism) in use.
The numerous holes down each arm are used to get the best clamping arrangement based on the size of the frame being clamped up.
I don't seel a hole in the base piece (where mine uses a piece of all-thread and a couple nuts for clamping pressure), so I'm assuming you used yours with a standard 'C' or 'F' clamp.
For typical use, a piece of waxed paper under each corner block will keep glue from setting up on the arm and making the frame a bit tough to remove. You will also notice that Ed's has a small radius hole in the center of the corner block cut-outs. This is to give glue squeeze-out someplace to go (again -- so yer not glueing your frame to the jig). Mine is missing this option, but hasn't caused a problem yet.
The jig is great in use, by placing the frame members into the jig and then holding the two base pieces (the piece the two arms connect to), then applying pressure to one side then the other -- the frame members will 'center up' and you can easily see if you have a bad miter that needs to be re-cut.
All in all -- if you plan on doing any picture frames, you need one of these jigs.