'57 was the last year for coaxial power seering and '58 was the first
year for constant control. I'm not sure what all the technical
differences are, but they sure advertised a big change. The '57's use
ATF and the '58's use PS fluid.
--Roger van Hoy, '55 DeSoto, '58 DeSoto, '42 DeSoto, '66 Plymouth, '81
Imperial, Washougal, WA