According to my 1951 HJ parts book, p/n 209960 fits both 4 and 6 cylinder cars. It is noted that the spring has 9 1/2 coils.
If yours has 8 1/2 coils, that's p/n 212586 and was used on the export/heavy duty cars. The parts list issued in November 1952 for the 1953 cars notes that the export/heavy duty spring was changed to p/n 213718 and now has 8.26 coils. Again, there is no difference between 4 and 6 cylinder cars.