On the plug question, the configuration of the prongs is the same for both 208V and 240V. It varies by amperage, i.e., you can't put a 30A plug into a 50A receptacle. The plug on the cord in the picture looks like the proper 50A plug, just an after-market replacement.
208V service is very common (but not universal) in commercial settings but sometimes is 3-phase and sometimes is single phase. 240V is always single phase. It is highly unlikely that you would have 208V service in a residential neighborhood. If a kiln is wired for single phase, the only real difference between 208V and 240V is the resistance of the elements to generate the same amount of heat inside the kiln. 208V elements can be run on 240V circuit of the appropriate amperage (which Neil covered) - it will just heat a little faster and the elements will burn out sooner. 240V elements will work - badly - in a 208V commercial setting. They just won't get hot enough to reach cone 6; cone 06 probably, but not much higher. The solution is easy, just replace the elements with ones designed for the available voltage. If you change them, it would be good practice to mark on the rating plate the new voltage so nobody is confused in the future.
A+ answer - This is perfect. My plan is to test it when I get it - if all but 1 of the elements work I might replace just the 1 at 208 - then when the rest burn out replace all with 240. I've got jenken about 30 minutes away from me. You think I would be able to use their elements? I wonder if they are the same configuration - could save some $ and time that way. anyway this forum is amazing.