numbers and class question ???

for awhile now i thought i knew but than realized that i really didnt know what the numbers and letters ment on the sides of old salt flat cars like the rolling bones have numbers and letters what do the letters mean cl*** of engine ?????

 

btt

 

Any answers depend on year, type of car vs rulebook of a specific time, the organization writing the rules, etc.
Normally you have a car cl***ification (roadster, modified roadster, etc.) and then a cl*** based on engine size.
All of this stuff changes, sometimes radically, over time. The same car would sprout new letters and numbers if cl***ified in 1954 and then in 1962...