The 1972 220D that I just purchased from the original owner came with a tool kit, which included this very allen wrench. I had already bought a 9/16" allen wrench, which would have required a small amount of filing down on all 6 faces to do the same job.
As someone posted a while back on another forum, be sure that you remove the "fill" plug first, before draining the oil. You wouldn't want to empty the differential, and then find that you cannot refill it. Use a piece of 3/4" galvanized pipe as a "cheater" to get enough leverage to turn the wrench.