This bike came to me with a reputation for tank-slappers, and sure enough, the head-set was iffy. Tightening it caused a strange self-centreing action, and loosening it until that problem was no longer evident meant that the head-set was too loose to ride it safely. The test ride was very short and very disconcerting.
When stripped for inspection it became clear what was happening; two errant balls had obviously been dislodged the last time the bearing was assembled, and had lodged up at the top of the housing in the old hardened grease. In this position they were preventing the cup and cone bearing from being adjusted and appear to have been happily wearing away the housing (see additional bearing wear like a huge smile beneath the race).
Given the depth of the groove worn, this has been like this for many years, so I wonder how many previous owners and riders have thought "this Douglas handles like a pig, I'm going to sell it on"?
Equally the ace mechanic in the dim and distant past who left it like this can't have much "feel" for how the steering ought to have felt after regreasing or whatever he did.