The part number for the chromed bar type adjuster (labeled adjusting nut) is 4670-34 and appears in the 1939 parts book. This part number also appears in the 1947 parts book. The Bakelite knob is therefore likely to be part number 4670-40, as this number would follow Ariel's normal stance of keeping the main part number the same and only updating the year suffix.
Regarding the dated 1940 sales catalogue ....I believe the 1940 models (as was the norm with the release of the next model year) would have normally been released in September 1939, so Ariel no doubt would have had the 1940 catalogue go to print a couple of months before War was declared on 3rd September. It shows the Torpedo rear lamp, so it would appear that Ariel intended to carry this lamp over from the 1939 year model and shows the 'bar type' 4670-34 damper adjuster, so no evidence that the Bakelite knob was intended to be used on the 1940 civilian model.
Looking at the 1940 / 1941 WNG parts list it does list the adjusting nut as 4670-40, although the picture shows a basic 'nut' as opposed to a bar type adjuster or a Bakelite knob. Such a nut would have needed a spanner to adjust and the serrations machined on the rear. A nut which needs a spanner to adjust 'in the field' does not really make much sense. So is it a case that the artwork is incorrect and that the Bakelite knob was intended as an WNG addition.
The sales brochure that is shown on the website as 1946 (with a rider that it is not dated but believed early post war) clearly shows the Bakelite knob on all models. It also shows the 1939 /1940 Torpedo rear lamp. The artwork has therefore clearly been updated since 1940 and clearly shows the knob shown in the Ebay listing.
The War did not end till Autumn 1945 so it is unlikely that Ariel produced a 1945 brochure. The undated (shown as 1946) brochure cannot be a 1940 brochure as we already have a dated one of those, and it shows girder forks whereas the 1947 one shows the newfangled telescopic forks. So the assumption that it is 1946 seems to be supported.
The Bakelite knob is clearly a rare anomaly but, given that someone has a few of them, shows that it was produced. Hopefully someone in the club will be able to enlighten us.
The WNG parts book gives a drawing number of D10/175. I wonder if Paul Jameson or anyone else who has access to the Ariel parts drawings can see if this is included in those.