A friend of mine let me copy these snapshots from 1970,on the back of the first on is written "paid £4 cash Redditch 1970",the next two show its transformation into a bobber,which I think was quite common in the 1970/80s,all its military parts and other parts that were not used on the bobber were scrapped as in 1970 they were of no value at the time. I wonder if NAU 20F survived the seventies.
Go on to the DVLA site to see if it is SORN'd, if it is, somebody still has it in a shed/garage/under a tarpaulin somewhere. Not being registered does not mean it is not still around, just not listed and now no longer owning that reg.