The ignition key is unique to the ponton. Have you read the following:
http://www.mbzponton.org/valueadded/other/keys2.htm
All ponton ignition keys are the same profile except for the 190SL. I have a very few of the original Neiman key blanks for pontons available. They are available nowhere else in Europe of the USA.
Contact me separately if interested. Not inexpensive.