Not that exact one, Dave but we can be pretty sure of the style used..This was just after the deletion of the white-edged black background but the style of number remained unchanged.
The figures were 1" in height, as per this image of my handy folding steel rule against a French-registered M20 that was at Beaulieu some years back...It shows the details of the hand-painted signwritten figures.
Hello Scott, my file of BSA photos is not as comprehensive as my records for Nortons and there seem to have been differences between the manufacturers.
With BSA, I have very few bikes with a local authority registration not having a black background and none of those without the 'civvy' number seem to have the background so it's likely to have been around the outbreak of war...September 1939...I'd expect KM20s to have had a black background and WM20s not to have had. It's difficult with the earlier bikes as the backgrounds were sometimes overpainted during 1939/1940.
This is the highest serial number that I know featuring the pre-war panel.
It's more complicated with Norton as they seem to have stopped in September 1939 and then started again in December, possibly until as late as March 1940 and I have no explanation for it !
As for later styles, they again varied between manufacturers and contracts...The best option is simply to look at photos on each side. I don't really have enough photos (if we think that there were getting on for 200,000 BSAs and Nortons) to be able to say that a style definitively changed on a certain date.