A possibility that occurs to me is that it might be a factory upgrade of an earlier camera. In that era Leitz offered a service whereby an earlier camera could have certain upgrades performed to make it function like a later model. And sometimes this involved changes to body parts (like top plates) and I believe, serial numbers.
For example, I used to own a lovely 111a that had been upgraded in the 1950s to give it the flash sync characteristics of a 111f, including not only a flash sync port but also an adjustable collar around the shutter button, like the 111f models, to change the flash discharge time for different flash bulbs. (Some bulbs were slower burning and so had to be triggered earlier so they were at full strength by the time the shutter opened). This is but one example of the sort of quite extensive changes Leitz very kindly made available (at a price) to their clients.