Registration Date

[JonE]

I'm pretty sure in those days the car tax year was the same as the calendar year. the current system was introduced fairly recently (shows how old I am!) to initially remove the pressure of new year sales and then latterly boost mid year sales

but its the fiscal tax year that would the possible determinant then, as the reg letters wouldn't have changed in the 1930s...
Garages may have registered cars at beginning or end of tax year to favour their tax return.