The aftermarket and classic car parts business is huge.
You may be surprised by the number of common parts shared by many cars. That's why car manufacturers bought many of their parts from 'specialty parts' oem's (like Bendix, Budd, Essex Wire, Holly, Kelsey-Hayes, Monroe, Thompson Products, Timken, Trico, Tung Sol, and thousands more).
If you consider Ford's 1959 production numbers, no single company could possibly produce enough leaf springs. Ford sold 1,684,108 cars (excluding trucks) in '59. Each car uses two rear springs. That's 3.36-Million rear springs. Each DAY (M-F) someone made 12,955 springs just to satisfy Ford's assembly lines. GM needed twice that number. What about Chrysler, American Motors, Studebaker-Packard, Hudson, Checker, aftermarket, all the trucks, etc.
Now you know why the Japanese auto companies want the US & Canadian market share. Mass production numbers are staggering and so are the profits.
Am I surprised a spring vendor sold ten sets of springs in a week? Not at all.