It took me a while to remember this one, but there's a tradition that supposedly started in British live theater that if an actor played two roles, the second role was listed in the program as being played by Walter Plinge, a fictitious name. This doesn't mean much, except the concept expanded a bit, and was sometimes used as an alias by an actor who was, maybe, wanted by the police or had deserted his family, or the army, or whatever, or just wanted to pursue an acting career with no questions asked. This means a little bit more since it turns up as a tidbit of trivia in a couple of Laurel and Hardy shorts: The safecracker in The Midnight Patrol and Hardy's butler in Me And My Pal were played by an actor billed occasionally in other movies, not those two shorts, as Walter Plinge. Thus the Walter Plinge concept may have come across the pond with Stan Laurel and/or some of his expat British friends.
There are holes in this story ( I got it off the Laurel and Hardy Forum, where it is discussed glancingly, offering barely any more detail than I've given here, and only as part of a sentence or two about that safecracker actor who, it is implied and not, unfortunately, discussed, led a very interesting and possibly very shady life ) and if anybody has got any knowledge that would flesh this out I think it would be very interesting and entertaining. One final tidbit that I have is that in the final credits of a Robin Williams movie, I don't know which one, one actor in a minor role is listed as Walter Plinge.