The 550 is the lowerst trim level for a Classic (770 is the highest). What this means, typically, is that the 550 will have less chrome, less interior appointments, and there may also have been some other items that were standard on a 770, but only optional on a 550.
AM radio outside adjustable mirror 195.6 split-bench seats
That's it. Rubber mats (no carpeting), no tinted glass, heater, 3-speed column manual trans (T98) (it broke) not sure if it had iron or aluminum engine as I had it yanked at the junkyard I got the car from.