It looks to me as if there are at least two obliterated numbers after the four. The number font looks typically continental. The barrel flange also appears to have had a grinder across it.

Most European countries had very restrictive rules relating to importation and registration of foreign vehicles during the post-war austerity period and it was not unusual for owners of ex-WD stuff to restamp an identity often using papers from a pre-war bike that had been lost during the conflict or just worn out.

In my student days I rebuilt a AJS. The post office told me I could change the engine or the frame, but not both obviously.
I changed the frame to a duplex one week and two weeks later changed the engine....

We used a similar trick with the spare bikes.
Swapped the engines over .
Got pulled over loads of times but Mr plod never twigged there were two bikes.
"Those clots in the office have got it wrong again"
We don't have log books so it was easy to do

email (option): bsansw1@tpg.com.au

I actually got all the details entered on to the logbook with this wheeze. Was more like a farmgate than a loop hole.