I calculate the WD serial number for WM20 38598 as C4355190. There were certainly some numbers fairly close to this that were sent to Australia. With a bit of luck, Rob Miller will be along shortly and can tell you from his databases if it had an Australian serial number, or if it appears in the post war British Army of the Rhine records which is the case with most of the WM20s in the U.S.
Thanks everyone for replies. It’s good to know some background on these bikes that are steeped in history. Looking forward to getting it back together and riding it when the weather warms up. Thanks again.