Looks like the main guide threads into the mounting block. A steel rod that fits through the cross hole would be used to turn it, being on the lookout for loctite. An allen wrench would work, but will probably make a burr that you will have to sand down. You could wrap an allen with a few layers of electrical tape and try that, but back off if you think it will deform the hole. You might have a round screw driver shaft that is the right size.