I don't know, I've never seen one. Just guessing, I'll say there's probably a pair of screws holding the (looks like brass in the photos) top plate on the box. These should be visible and accessible through the battery door with a flashlight and a long tool. Remove the screws and the top plate should lift out, fall off or be pushable from underneath. You'll probably find the switch, battery springs and positive current path-- however this is arranged-- mounted to the back of the top plate. If the mod is a true mechanical, the "switch" won't be an actual switch, just a spring loaded actuator that pushes a piece of conductor into the positive current path somehow. Maybe your spring is broken, or perhaps juice leaking down the 510 has gummed up the actuator somehow.
Be very careful. A stuck button on a mech is seriously not funny. Depending on your atomizer resistance, it might result in a thermal runaway and a venting battery, or worse. I don't see any vent holes in the case (shudder), a situation you may wish to remedy while you have it in pieces. I would also caution you to be sure you're putting the batteries in the right way every time. Reversing one (in the absence of individual battery fuses) would also be highly unamusing. No offense, but that thing doesn't look very safe to me. You might be better off junking it and getting a Hammond box with dual parallel MOSFETS.
