Thomas, I am thinking of the AIACR engine classes (introduced in 1925?) which I assumed the FIA inherited in 1946. Nothing to do with SCTA.
Alright, more information for better understanding
Four wheels vehicles was split into two base categories....
A = special construction....like speed record racer
B = production vehicles
in 1965 they add
C = thrust power - two different type of engine....JE = Jet....RT = Rocket...
A and B stayed for special and production....
but the type or kind of engine was split
I = charged fuel/gas engine
II = N/A fuel/gas engine
FIA makes no different between gas and fuel....
III and IV = diesel engine....
VIII = electric - split in three different weight classes...this changed last year to more weight classes...I think now 12
IX = turbine engine - split, like electric in weight classes - three - and now to more
the piston engines using than the 1 to 11 for the engine size....1 is the smallest, 11 is unlimited
before the engine size was in letters...Frank Lockhart run the Stutz Black Hawk in size D as an example......
hope this helps to understand this categories and classes world inside the FIA record rules...
it could be around 1925 when the start to use this new vehicle classes (A and B)....this was also the time when the short distance speed record vehicles became special construction....before they was regular street cars at first, than regular used race track cars, so the Sunbeam Tiger....Campbell's BlueBird (his first own built, not the 350 hp Sunbeam) was the first just for record breaking built car...and so this first bigger chance in the rules makes sense....