No idea, I do have an old rheeme ~10l keg that was re-purposed as a compressor expansion tank. Handy for topping up the tyres on the cars that are out the front.
I wouldnt even think of trying to convert it back.
Looks like it may convert to a horizontal stillage real ale cask easily. What size is the thread that is blanked off would it take a plastic cube/fermenter tap? If so vent through the compression fitting?
I think this ball lock post from ibrew designed for a bulkhead will work, preferably using a 1/4 inch to 5/16 female thread connector (does such a thing exist?) and using existing port to save drilling new holes.