this is as well explanation which i heard, to avoid paying tax. Leitz (and maybe others) were doing it already before WWII. In particular, almost all III(a)sync originating from I, Standard or early II resulted in new camera returned back.
When talking about conversions - this was my favourite collection topic until few years. Some of my most interesting example I was showing in Leica Forum, German parrt, thread Leica Umbauten.