Registration numbers in order

[morrisminor]

I've been doing some research into my Ruby CTT 290 and came across a list of numbers in order,  it's interesting to see the variety of cars.
CTT 289   Austin 7.8
CTT 290   Austin 7.8
CTT 291   Morris 8
CTT292    ?
CTT293    ?
CTT294     Bedford coach
CTT 295    Austin 10
CTT 296    Austin 10
CTT297      goods
CTT298     Morris
CTT299     Standard 12
CTT300      Ford 8
CTT301      Bedford
CTT302      Lanchester 13.4

Good variety, any idea what the 7.8 means for mine  ?
TT is Lynton and Lynmouth, North Devon
April 1937