Those letters represent the NHRA classification for those particular cars.
SS stands for Super Stock. That would be an eliminator category. An eliminator consists of a group of cars adhering to the same basic rules.
The E or F represent the specific class that the car fit in.
Super Stock cars are classified or factored by factory shipping weight vs. advertised (or re-factored) horsepower ratings.
Super Stock required all stock body panels and full, stock interior along with original carbs. For example: a light car with high horsepower would be classed closer to "A". A heavier car with a lower horsepower rating would fit somewhere down the alphabet.
MP stands for Modified Production. That Eliminator category allowed for certain non o.e.m. parts to be used such as hood scoops, non original carburetors, etc.
Modified Production cars were classified by actual weight vs. displacement. An A/MP would weigh less than a C/MP car providing they had the same displacement engine. Modified Production went the way of the dinosaurs in 1981.
You could spend a lifetime studying and learning this stuff!
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