Yes, putting a bolt or pin through would lock it just as well (more securely, actually - every once in a while one of the arms will bump something or get caught on a rough bit of ground and pop out of the catch). A bolt was going to be my first design, actually, but then I thought of doing it this way. The primary advantage of this is I don't have to bend down to put a pin in. I can just step on the arm, which lifts the tractor, and push the catch plate over just a bit to latch it under the lag bolt. Move the tractor, and use your foot the same way to unlatch it and set it back down. That's why there's an extra chunk of wood attached to the front of the arm, to give more area to step on.